Chances are
MonsieurX is correct. It isn't so much the game is messing with the scheduling as I suspect you are picking one girl and concentrating on finishing each of her achievements. Not that there is anything wrong with doing it that way, but Yvette's strength training was one of the first extra achievements for her (can't recall what version# of the game it first appeared) and the game is running through available scenes/events/appointments and just triggering whatever is available. Then again, I might have no idea what I am talking about. If
darkhound1 sees this post maybe he can offer an explanation.