This. I dunno about in America, but just because five is in the middle, doesn't mean it's an average score.
That's a D in the UK grading system at school, so why confuse it with scores?
No-one wants a game that is a 6, regardless of your grading scale. You want a game that is an 8 or above, 7 if you're a fan of that particular franchise and are willing to take one of its weaker entries.
The way this scoring system works is that 0-4 is an almost impossible score to give a game, 0-1 is basically a broken game, like Big Rigs, 3-5 is a game which is playable, but only if you got it for free. 5-10 is for games where you paid money and expect a game of some sort. Just like with exam scores, where 0-50% is "you need to do this subject again, because you basically didn't study it£" and 50-100 is the "how good are you at this subject" score range.
And Edge score of 6 doesn't mean they're using the "right scale", that some people are talking about. They're just quite notorious for having quite critical reviews. That was their reputation back in the day anyway? They don't give out that many 10s.