There are a few issues here.. There's the reviewer side, and the fan side. Most reviewers, as I'm aware, review games on a 1-10 scale. The reason you see a high average review is because of the game selection (reviewers very often don't review shitty games since their audience doesn't care about them either).
Then you have the fan side. From what I've seen, they're the ones who are interpreting the scores the way you described. They're the ones who think a 6 is utter shit and unplayable and cry when their game receives that score. I don't think it reflects wrongly on reviewers but rather the gamers.
No one wants to see their favored franchises reviewed less than competitors. Hence the cry from gamers when their favored franchises review in the 70's or lower.
As far as a correlation between review score and unit sales go, there has been proven by data before that there is one. Higher reviewed games perform better. This isn't supposed to show any sort of causal relationship, but rather shows that review aggregates do seem to correlate with what consumers enjoy. That shows that reviewers are doing a measurably well job at what they do. At least when you average them all together.
( I just re-read your OP and I think you actually agree with my points. If so, I'll restate again that the reason I think there is such outcry over moderate review scores is because fans want to see their franchises doing THE BEST, and they flip out at the thought that the things they prefer are "just okay").
