Is YMMV just a provider's way of saying that even though such & such is on the menu, it doesn't mean you may partake of it? is it a polite way of telling the hobbyist that there is no way in hell i'm doing that with you? I'm just curious, not judging. As far as i'm concerned, the provider can do as she damn well pleases.
It can mean come in without a shower and may not find BBJ
Particularly with Greek, which was mostly a size issue.
But also, anyone who tended to slam away at Door #1 was not allowed in Door #2. 
Some ladies will kiss, but only with tongue if she's feeling it with the client. Or she doesn't want to commit to LFK/DFK for any number of reasons. :shrug:
We aren't just giving haircuts here; there are a LOT of very personal variables which lead to your mileage varying. LOL. 
A very eloquent way to put it.
...just because she did it with the guy in the review you're reading (and/or it's in her profile) doesn't mean she will do it with you. "It" could be anything. Debra and others have given good examples but it isn't restricted to those examples.
... P411: "Your Mileage May Vary: a reminder that providers, like all women, have their own likes and dislikes, therefore experiences vary."
Just going by some of the examples I've had thrust into my personal space at the gym (stinky guys getting within 3 feet of me and overpowering my admittedly sensitive sense of smell), I can't IMAGINE how providers put up with some of the gross stuff that we reportedly hear about in the discussion boards.
That way I'm always satisfied with the time I had, regardless of what the provider is in the mood to do or not do while I'm in her company. For example, I have read reviews where several claim that the girl will swallow everything, however she is clear that she does not want that. On the other hand I've seen providers that the reviews state she does not swallow, or even does not engage in bbbj, and yet she did with me and when I warned her of the flow coming forth, she didn't even pause, just kept on going.
In my case, YMMV is usually a winner because I keep expectations at their lowest going in, and am often pleasantly surprised.
Most guys put that in the reviews because they are not sure if what was offered was standard or not. Just a respect thing.
If a gal advertises as such, it is because it depends on how you are or how your dick is. It doesn't even mean everything is YMMV, it could only refer to one thing, such as Greek.
If guys treat a girl right and have proper hygiene, they get pretty far, farther than someone who doesn't.
You really want me to linger or keep up what i am doing? Massage me, touch me lightly. kiss my neck. Keeps me there longer. Plenty of ways to get what you want by being friendly to her.