Check out this thread...it's just a little further down the page.
Or did you see that thread and decide to start this one to get some extra posts?
I think it really is a mind game. Betting standard multiples is just that, standard, and almost subconciously I think throwing a wierd looking bet throws a sort of curve out there. That, and perhaps the player has a specific reason they don't want to bet x amount. Perhaps they feel 3 times is too much and 2.5 times is not enough and so they choose an amount in between.