You shouldn't see the number of 90% om steam as a indication that it's a 9/10 game. Just that most people like it. Its kind of the same issue with Rotten Tomatoes. The score is not comparable with the average score on say metacritic or a regular review.

It's the number of people who recommended the game in a review (in the case of RT it's the number of reviews that gave it equal to a 3/5 or better). And it seams like Steam users tend to give recommendations unless the game is broken (or the devs did something they don't like). So a game that everyone thinks is worth playing but not fantastic will get overwhelmingly positive even if nobody really loves it.