It depends on the reviewing site. And even then, it depends upon the reviewer writing the review. Some are harsher than others, and sometimes a game just doesn't click with them.
A 6.0 on Gamespot is considered "fair" which is pretty average. A 7.0 is considered "good," which is above average, so maybe Conduit didn't earn a 7.5 by Gamespot/reviewer's standards after all.
8.0 is "great." And a 5.5 is "mediocre," which typically means skip unless you're buying it for the characters or license, or there just isn't any other game in that niche to compete.