When I see "YMMV" listed as an activity enjoyed on P411, does it mean that a provider just lazily copied every acronym she saw and added it to the list, or is it a warning that not all activities advertised will be necessarily be offered?

If she wants to give you a BBBJ, she will, if not, she won't.  That's the sort of thing YMMV means.  She will give some guys certain services, and other guys different ones.  So read plenty of reviews!!!!!

Swim

Some providers need a certain chemistry in order to perform certain services.  Also hygiene is really important to most escorts, and I'm talking yours.  If you're not squeaky clean she may decline certain activities. And size also matters I'm talking both your equipment and overall size, especially weight.
Equipment to big, no trips to Greece, overall size humongous and certain positions maybe excluded.

It also means that she does not a reviewer posting all the details in the review of those special services. I saw one the other day that gave DFK and daty. She told me not to post in the review and she does not care if it makes her score lower. It is best to PM the reviewer to get the details.