I don't believe numbers can represent what a game has to offer.
That being said, a 10/10 doesn't necessarily denote a perfect game, whatever that is. It's a critic's opinion that said game is cream of the crop. When critics were handing a 10/10 to Half-Life, they weren't saying it was the best game ever,(well, some of them weren't) but that it had more to offer, qualitatively, compared to other games of the generation.
A Stone Age cave painting isn't as sophisticated as the Sistine Chapel ceiling, but that cave painting was a pinnacle of art at the time. It was a 10/10 that seems primitive now, like comparing Super Mario Bros 3 to Bioshock.
When Halo:C/E was given 10/10, was it because it redefined the FPS genre? No, it just did it better than everyone else. The game wasn't perfect and I believe no game is. A 10/10 does not mean perfection, merely a high standard of gaming.