Well, for a game to average a 5 it would have to be fundamentally broken on some level, so that's hardly surprising. Scores below 7 are generally reserved for titles that have serious technical issues (graphics, UI, controls, player feedback, etc.), and as such are badly made products.
The real review range lies between 7 - 10, and that's where the disparities are found. Also, the higher up the scale you go, the more each percentage point is worth: the difference between 90 and 96 is much more than the difference between 70 and 76. So a score difference of even 10% can represent a pretty huge difference in quality. For FPS, 80% is the low-end: even shooters that are received relatively poorly by the fanbase can get around that mark. On the other hand, 90% is the absolute high-end, reserved for AAA titles.
So that 10% margin of error is actually pretty big.