Most likely (I don't know for certain) the world/country ranks are an ELO-based system. In which case if you're forever playing "low ranked" teams in the meat of your schedule (your league games), you're at a natural disadvantage to a team who gets to play a higher average rating for their league games. So, USA teams "prop each other up" because they have higher world rankings in general, so a loss doesn't cost as much rating and wins net more rating. Teams from less prestigious countries play games against generally very low rated teams, so they get little-to-nothing for league wins and suffer cataclysmic rating losses if they drop a league game.