I am sure thhis has to do with the dollar value. Brits use brit pound so USD isn't much. I think Canadian players are pretty serious actually. There are a lot of forums based on poker in toronto and meet ups, etc where people are actually good.
But I think German players are probabaly the best. I used my old PT stats and I see anyone with a name "zuburffhweh" etc have won some from me. Its a small sample but works for me when I avoid them.
But every country has the sharks and the fish so i don't think it is an accurate measure. In third world countries I think there is generally more cheating such as collusion, etc.