First of all, what's that meme?
Secondly, it's because it's a proven baddie tell. Of course it's not 100% accurate, but it's pretty good. The reason for this, I think, is that baddies want to have as many backup options and excuses as possible. If they vote for an important goodie, they can always say "well, I wanted to change my vote, but I was too busy!" or something along those lines.
Goodies usually don't bother to say they will change their vote later, they just vote and change it later if they have changed their opinion.