Exactly. In order to get anything of value out of ratings in the 6/7 range, you need to know and be comfortable with the scoring and review history of the outlet you're looking at. You can't assume a 7 is mediocre or average unless you know that said outlet has a history of scoring games like that. I've generally used EDGE since 1997. In the past, I could buy pretty much any game they scored a 7 or higher and know it would be good. I also knew that some 5s and 6s were worthy of my time.
In short, my advice would be , find an outlet you like and stick to it. Don't compare apples with oranges. Don't assume a reviewer values the same things you do. Above all, read the text and form your own opinion. Oh, and don't be pissed at a score that an outlet you barely read gives. Be pissed at the text of a review of you want a debate. End rant.