If I understand OP correctly, the premise is that the average skill of players in NA is lower, so if you actually manage to clear high end content, you tend to be pulling more weight with you, whereas in JP, since the average skill level is higher, players don't need to be as good to clear content since they can rely on other players to carry more of that load?

Is that the general idea?

That is, if everyone's a 7 out of 10, you can be a 6 and reliably clear (JP), whereas if everyone's a 4, you need to be a 9 or 10 to drag them across the finish line (NA)?