The reason a lot of girls start with a very high rating which then goes down is simple, and goes like this. The first few ratings will have come from people who have BOUGHT her card, either because they like her type, or because they already have card(s) of hers which they like. They will be few, and their ratings will tend to be high because they're a biased group. Later ratings will come from people for whom the card came up with the rations. Some will like, others will be indifferent, others will say, 'Oh no, not HER again!' There will be more of them, and the average will drop, often sharply, then plateau-out.
The idea that people will pay multiple subs under different handles, just for the sake of down-rating girls they don't like is pure paranoia. As for ratings by people who don't own the cards, I'm against it. No representation without contribution!