Providers must hate the 90 minute session, why else would they price them higher (on a per minute basis) than the hr or 2 hr sessions?  I have found this quirk in the prices to be almost universal and has been true for all of the two decades i've been hobbying.
Here is an example from a very large and popular agency in the DC area:  
APPRECIATION
15 Min 200
HH        350
1 H 400
90 Min 750
2 H 800
I understand the price for short sessions being high (on a per hr basis) but usually when one buys something, the price per unit goes down as you buy more units.  For example, if you buy a single Coke it's 1.25ea,  But you can buy a sixpac for 4.95  (i.e. .82 each), or a case of 24 for14.00  (.58 each).  But with pussy, it is 200 for 15min (800/hr) then 350 for HH (700/hr)  then 1Hr for 400/hr... so far it makes sense the per hour cost goes down as you buy more time.  But then the 90 minute session is 750, which is 500/hr which is higher per hour than either the 1 hour or the two hour session... and why is the two hour session twice as expensive as the one hr with no discount for the second hour?
Providers:  Is there something about a 90 minute session that you dont like?  Are you trying to discourage 90min sessions with this pricing?  Am I missing something?

I don’t see this issue with myself. The more time booked the more value for the price. I prefer longer encounters & that’s why I offer specials for dinner dates, overnights & weekend getaways. Why not ask the providers who you know do this?  

Many providers like longer dates, so they set their rate schedule to encourage guys to book longer dates.

Very true. The half hour is priced high to incentivize the monger to bump to an hour. The 90 minute is priced higher to incentivize the date to two hours.

I think 1.5 hours is the perfect amount of time: it gives you anywhere from 5-30 minutes to get acquainted, relax, have a drink maybe, and then you still have 60 minutes of playtime. That's my usual style. If a provider has a "preferred" booking period that's fine and maybe that's not a good fit for you.

I suspect shitty math on the example given, and most others that have their 90 minute rate "out of whack". Especially when the 2 hr fits the scheme of more time results in a lower per minute rate. I'd say about half the girls "get it closer to right". Some ladies seem to encourage 90 minutes on a $/minute scale.

something called "Hooker Math."  
Which means some girls aren't too good with numbers and create rates that make no logical sense.

You're overthinking it.  My theory is that it's to make the math easy.  Providers price a half-hour (A) and one-hour session (B); then 90 minutes are A+B, and one hour is B+B. There is no multiplication, just addition.   In many cases, there's nothing more to it than that.  

In your first sentence you stated this is an agency, not independent providers, so you are comparing apples to oranges. A provider that works for an agency does not set the price structure, that is done by the agency.  
The structure is created to make the most money. The agency does not want you to select the 30 min or the 90 min, they are incentivizing you to go with 1 hour or 2 hours.  15 min dates are obviously going to be popular (and most profitable) with guys who want a quickie during their work day and don't have a lot of time.  
I personally offer 60 min or 90 min for a new date, either is fine with me. If things go well I offer longer date options.  
So nothing special or magical about the time options, it is simply a business trying to make the most money possible.  
I am going to assume just by looking at this price structure without any additional information this agency's most requested booking time is 15 min followed by 1 hour. Very doubtful guys ever book 90 min or 2 hours. Some may request the 30 min option, but the booker is going to encourage them to go to 1 hour for only $50 more. So, business is business.

And they never have.

to discourage doing 90 minutes.  Guys who book 90 minutes generally want two pops, but the first pop takes 45-min to an hour.  This makes the second cup rushed, resulting in a couple of things . . . . 1)  the girls has to work harder to finish the second pop in whatever time is left after the first pop went overtime (meaning it used up MORE than half of the 90-minutes, and the second pop traditionally takes longer) and they have to work much harder to finish the second round before time is expired, and they want to be compensated for the extra work, and 2) since the second pop was rushed, the customer is leaving less happy with the session than he would have been had he gone for two hours, which means the otherwise excellent review may be downgraded, which is an added risk for the girl and the agency.  Consequently, the agencies do everything they can in order to push you up to 2 hours, including making the pricing for 90 minutes unattractive from a value standpoint.

One hour is B not B+B

I'm pretty sure you broke your rules for me or is my brain on a break?

Of course I broke my rules for you!  
And it was worth it, of course.  

This is exactly it, it's a business decision to make money. To elaborate, this is an agency trying to make up for opportunity cost of the 90 minute appointment. Let's say they have their girls working from 10am to 9pm. 10 hours a day. Most guys will book 1hr, probably like 80%-90%. If you accept a 90 minute appointment, you have to hope that someone else that same day books a 90 minute or 30 minutes appointment. Otherwise you now have 30 minutes of free time for the girl that she isn't working and not making money for you. So the extra $$ for the 90 minutes is a way to make up for some of the potential earnings loss.

Yes - 2 hours is B+ B.  Oops!

Ninety minutes is longer than 60, so one would then expect 90 minutes to be priced better than 60.  The OP says it's usually worse.  So your suggestion contradicts the (alleged) facts.

I don't really worry about the pricing structure offered (I assume it's working for the girl/agency if it stays and if it isn't working that it would be dropped -- in any case not my problem to try solving) but do see that argument you seem to be missing. Do you really think someone that initially might be thinking of 30 minute session is going to be induced into a 2 hour session? No, getting them to go to a one hour session though is much more likely. Same for the ones that want a session longer than an hour -- moving them to a 2 hour session from a 90 minute session is possible.

Bingo Jensen! Think of it like a restaurant. They hate reservations for 1 person or 3 people. Their tables sit two people or 4 if you put two tables together. If they seat a party of 1 or 3 they lose revenue on that 2nd or 4th seat. It is the same way for some providers. Any half hour or extra half hour locks up an hour. If you book a half hour date, she can't book a more lucrative hour date. If you book a 90 minute date, she can't book back to back 1 hour dates or one 2 hour date. It is about opportunity cost as well.

I was on P411 earlier today and found the following posted price: USD 600 - 1 hour / USD 750 - 1.5 hours
That to me is the right price structure, if more ladies made it a "deal" to add the 30 minutes then more guys would choose the 90 min session.  Unless the lady is booked solid, seems like a win-win.

I mostly see ladies pricing 90 min sessions cheaper (on a per minute basis) than the 60 minute, although sometimes the price breaks don't start until after the 2 hour mark or more.  From a lady's perspective, unless she is really high volume, then spending more time with existing dates is easy found money.  This is why they always say on their ads and websites how much they love dinner dates.  More time with one guy will always be better than less time with many guys.  The amount of sex is typically less with one guy vs multiple clients for the same amount of time.  (Can one guy go as long or as many times in a 2 hour session as 2 guys can in 2 one hour sessions?).   Every new client is a risk - a safety risk, a LE risk , a bad chemistry or bad hygiene risk.  Trying to get guys to spend more for more time makes an enormous amount of sense for the ladies, especially if the alternative is sitting around watching Netflix or scrolling through TikTok.  

Better by spending more time . As for me as a client, I think 90 minutes is  perfect time  for a date - more than an hour but less than two, especially if a second pop is possible.