Well, I've noticed the same trend but I think there is a reason for that. It seems that when they are not on the tops list, its people who actually search for boobs, or are regular fans of that person that vote for them. Once on the tops list, anyone clicks on those (and I'm sure most people check those out) and are then voted on more than non-featured boobs. This increased exposure is probably what is lowering the votes.
Another reason could be that girls who want to be on top vote down those on top. Buuuuuttt I really don't think this is the reason. Not enough people to make a huge dent in that raiting.
I say explanation A!
I hope my explanation makes sense. I'm terrible at expressing my thoughts