Yeah, not only does relativity play a role, but some people rate the girls they can't have really high just because they can't have them.. think about it. I have been guilty of it myself. I knew an incredibly hot girl for a few months but everytime I was with her my attraction level for her actually dropped.... I found myself just brushing her off as nothing special even though when I was away from her I wanted her so much. On top of that, you have the girls who have horrible personalities.
I was macking on a cute girl who I before would rate as a 7 or 8, but after flirting with her, her personality totally turned me off. On the looks scale, each person will find a few 10's in his/her life, but if you factor in personality, chances are very high that it won't happen.