I've seen this a couple of times in reviews and while answering ?s in my own review...specifically as it relates to anal.
Your Mileage May Vary. It means you may not get the same services as another of her clients, for reasons known only to the provider. It doesn't always refer to anal, there are many services that providers offer besides that where one client might get a different service than someone else.
Swim
From kissing to BBBJ to massage, The OP did mention anal, and reviews. I believe he asked that because it might be the only place YMMV is an option on the profile. In the list of services the choice is ususlly "yes," "no," or "don't know," with a few exceptions. For anal the choices are "yes," "no," and "YMMV."
It means just because a review says she did XYZ for some other guy, she may or may not do the same for you, depending on countless variables but mostly driven (IMO) by your appearance, hygiene, and attitude.
in many instances the YMMV variable is "size."