GS doesn't consider a 5 to be a game of average quality. That's only what their score descriptions say, but what they say in their reviews often contradicts that. Aside from that, the average is the average. It's the 'medium standard' set by all games together. If a website truly considers a 5 an average game, then summing up all reviews and dividing them by the amount of reviews should be around the 5. This is clearly not the case. Same goes for just about every reviewer out there.
Read the mania: http://www.fanfiction.net/~nonexistinghero In SA2, it's Super Sonic and Hyper Shadow.