How is the rating determined?

Under "About" probably. Top right of the page ^^^

A bunch of manic-depressives sit around throwing darts at a board, while dreaming of Roger the Cabin Boy...

Camaris, ratings are given by people with higher ranks. Every one of them have their own way of giving the ratings. So your questions just really has as many answers as there are raters.

I myself only give a rating when I see a skin has a rating too low than it deserves. Then I usually give it a 10 to boost it up a little.

Think of it as being the skinning equivalent of the Internet Movie databse (www.imdb.com) which has the same rating system. It works pretty well. I won't go see a movie that gets less than a 6. Once the per section filtering is in, I will set it so that I don't see wallpaper that gets less than a 6 for me. Other users will be able to decide for themselves if they want to use it or not.