And again, if they think that Xbox games are on average tier of quality above PS3 games, than that's a negative bias towards the PS3. There is no way to justify it or explain it. The only option is that they either prefer the Xbox significantly, or they are using double meter.
I think I know what you're trying to say. But in my opinion, this is not what bias means. Let's assume that the PS3 library was ACTUALLY weaker than the 360's. If you'd point that out - would that make you biased?
Now if we consider the fact that there is no objective 'quality scale' for games, I'd rather put it this way: EDGE has it's own emphases, preferences and ways of judging a game. In this coordinate plane, the top 30 360 games seem to fare better than the top 30 PS3 games.
Bias would mean that they downgrade games purely for the fact that they are on the PS3, and you have no evidence for that, since you are comparing completely different games.
Currently playing: NSMB (Wii)