well, first off if you have a bigger subject then people vote for their favorite show or whatever not the quality of the sig...
Or not. If you ask people to vote for the best sig (quality wise), you may be suprised.
secondly people who have bigger subjects might get better pictures for their subject as compared to other people....
If the person chose their subject within a category, it's quite likely they all ready have a lot of pictures for that series (eg I could do either a good Kanon or Card Captor Sakura one. Bet you havn't hear of the first one
), so it's quite likely they have good pictures.