Probably it's a Gears-of-War-Setting-the-Bar-Too-High effect.
Or rather, a similar game at a similar time can't score too close to Gears. Same reason Gears couldn't possibly get anything less than great scores. The press has to obey the demands of the hype machine they create.
Hey, call me crazy. Look at it this way:
If MS had chosen LP as the 360 poster child instead of Gears, these game would be in exact opposite positions today. Critically and commercially. I'm not trying to convince anyone, but know that you can't convince me otherwise.