I've noticed this a lot recently... see a post with a bunch of favorites, score is about in line with the # of favorites (since favoriting adds 1, this makes sense). Come back later, and the score is way down.
Using the completely innocuous post #2183681 as an example. It's got 7 faves, score 3 (at the moment) - while it's not a fave of mine, I'm using it as the example because the art is fine and it seems completely unobjectionable. Are there, say, people just autodownvoting everything boatgirl because they hate how much of it there is, or is there some score mechanism I'm missing like a decay over time? I did some searching on wiki and help and forums and couldn't find anything other than the upvote and favoriting doing +1 and downvote and unfavorite doing -1.