Well, there are no official skinning guidelines. Nothing defines what one rating means. So everything goes, really.
Veterans will remember me for my big rants against the rating system specifically because it means nothing.  Because there is no guides, because there is no rating grid, the rating values are therefore 100% personnal and therefore anybody can define his own rule on what a 1 is or what a 10 is.  Mr X gives it a 2 because he hates the color, and Mr Y gives it a 10 cause it has animations and Mr Y Looooves animations!
Having said that, I tend to rate according to my own personnal tastes, because there was no other way put forth.  If I hate a skin, no matter how technically brilliant it is, I am incapable of giving it a high rating.
A while back, I suggested a rating grid. It is a bit more of a fair way to rate, IMO.