Providers unanimously would agree that a "rushed experience" is not pleasant. Starting from that premise, this rate structure indicates that she would rather a) have enough time to meet you and relax at the date b) that she is low volume and not a fan of one hour dates.  
I would say she loves 90 minutes, hence the incentive to charge 50 extra for 30 minutes over the hour instead of 125.  
A good marketing approach if you ask me, specially if she lives in a busy city where most would pay for the 90 and maybe stay the hour...

They may not be good at higher calculus but they know what they're doing!

Marketing vs Math

To sum it up short and sweet. It's excellent marketing. Liken it to going to the movies and buying the combo or the large soda/ popcorn. It is just more enticing than the small at a mere 1.50 difference.  

Although I do understand that with companions its not 1.50 it may be 50 100 maybe even 1,000. It is not cut and dry  math.

Marketing 101
Adrianna Azzure

There is no logic to it.  They are left brain dominant.  It's about how they feel !!!!

Jonathan

Maybe no worse than the general population.  

That provider may be trying to push guys to 90 minutes.

What I've heard from more than one provider is that they like even amounts:  nothing ending in $25, $75, or maybe even $50.  In the case you note, maybe she didn't want to charge $325 for 90.

A woman's grade in math is identical to her cup size.