They're also relative (and completely stupid). You can give Liar Liar a 9 if you believe it's a near perfect film of its genre because your expectations are most likely not the same as going into a Kubrick film.I appreciate that reviews are subjective, but have you considered that after giving, 90's Jim Carrey movie, Liar, Liar 9/10, you may have blown your wad a little early? I'm expecting a deluge of 10/10's in the coming month.
I don't give marks to films, but if I did, I'd most probably be doing it based on the genre and my expectations, which wouldn't mean that I believe that an 9/10 is necessarily a 'better film' (whatever that means) than an 8/10 of a different genre.