It may be that some women find a two-hour session more demanding. Perhaps not more so than two one-hour sessions back to back (though possibly even then), but still more so than two one-hour sessions with plenty of time to recharge between them, and most of the cases I’ve seen with price lists like this don’t look like they are trying to maximize the amount of clients-hours they work per day, and it may be that they would rather have a day with two one hour sessions with an hour or two between them than a two hour session.
Part of the job is staying ‘in character’, and for some people (certainly not all) that can get harder and harder as the clock keeps ticking.
On the other hand, it may be that they find the second hour more demanding because for most guys, each ‘shot on goal’ takes more ‘work’ to get off, and for some guys, it’s the woman who’s going to be doing most of that work. (Which may be why many also stipulate that if you are having a 3+ hour session, at least some of it is going to have to be spent outside the bedroom.)
These two considerations pull in different ways, but either one on its own might make someone want to bump the two hour price a bit.