Is it specific to the US that has the supposed bias?
Not trying to troll/discredit, genuine question.
Yes for the most part I believe so (And Japan I guess?)
It's pretty normal to have bias in this industry. EDGE give UK developers high scores because well, they have a British bias.
We were talking about this the other day that people who grew up on Sonic games can't get into Mario games and vice versa. This isn't some sort of massive conspiracy that Nintendo selectively controls all forms of media, it just so happens they had a decent period of dominance in the US, likewise I'm not saying certain companies don't get favourable reviews either. The majority of Sonic's high scores these past few years have mainly come from European magazines and sites, games like Sonic and the Secret Rings and Sonic 4 Episode 1 enjoy a decent metacritic thanks to how popular Sonic was in Europe. Normally you'd have a point if the spread was evenly match from one game to another but Spencer found European outlets usually gave SEGA an additional 8 - 10 points I believe.
(Yes you can tell me SEGA has a European bias and I'd have no problem agreeing to that. I've stated my position on this subject before.)