I treat it as the school grade scale.
9+ - A
8+ - B
7+ - C
6+ - D
5 or below - F
So anything below a 7 is iffy for me. Not that I wouldn't try anyways.
This is what I do, cept when I was in school, a 60 was an F
No games really get scores in the 10's, 20's 30's 40's so if your game ends up down there... man you really know it's bad. By this logic, you can kinda see where saying a 50 is average makes little sense. The word 'average' can mean 'good' to some people, and I think that's the root of review score discrepancy. Sorta like "Most games are average, so this one is as good as the rest of em". You're then saying that a 6 is better than average. Technically, yea, but I can't bring myself to think that a 6 is good score. It's still an F.
Games in the 50's 60's you can kinda take a chance on depending on your taste. 10-60 = varying degrees of garbage. 70 = average, 80 = good, 90 = great or excellent 10 = the best of its kind, or transcending in some form/hits that special personal place in your heart.
To be fair, I have played games with low review scores, and enjoyed them. It's not a regular occurrence, though.
To add, a 5 is simply the average number between 1-10 itself, that doesn't mean it's the average score of a game, with varying factors weighing into the decision. Someone would need to calculate the scores of all video games ever created to find the average score and create a non disputable rating scale... For example, the highest score you can get in bowling is a 300, but if you take the personal scores of 1 bowler, or even tried to find the scores of all bowlers in a tournament, or season, you'll prob find that the average score is not exactly 150. That's just simply the average number.
It's basically all about perspective on what you think the word 'Average' means.