Also.. since games and tech evolve the number system is faulty. I remember a reviewer writing that he regrets giving GTA4 a 10 after LBP came out.
What qualified as a 10 three years ago, would not be a 10 with todays expectations. Sure a game can be perfect at the time, and a new game can be perfect at this moment aswell. But then the whole metacritic thing is moot since its a way to compare games based only on their scores and viewed as something current no matter when it was released and what was possible at the time.