In my opinion, guys don't rate girls. What we're really rating is the intensity of our attraction to a girl. We form a rating based on the sensory information we get. A six standing across the street may not be a six up close.
if she's far away, then they are rating what level of attraction that guy feels for that girl/ whether or not the girl has nice boobs/butt. that rating could change upclose based on that guys particular taste. a six on a 1-10 scale usually means tht your a little bit above average. with five being average. hope this helped a little bit:)