Do 360 owners have too high of expectations?

[Picko]

Expectations drive all reviews. Cutting edge games from yesteryear are often uninteresting and uninspired games by todays standard as a result of expectational changes.

Here's a simple reason for your observation. The Xbox 360 has a greater library of games, therefore expectations are necessarily higher. Therefore you see a superior library leading to slightly reduced reviews for newer game.

It undoubtedly has nothing to do with reviewers deliberately trying to hype certain games. 1% increases over another consoles release are not going to have any statistically significant effect on sales.