This whole north/south thing is really odd, because the Elves have traditionally associated the North with evil (when you go through the cardinal directions it's supposed to go West, South, East, North because North was the seat of Angband).
About the best correlation I can make is because evil was such an obvious and strong presence, some of the Men (the first ones to move west) reacted to it, and continued to live in the North in opposition. In the south it had a more subtle presence and thus was able to ensnare more Men. Or, those Men who did not react against Morgoth had to move in south, because all the good lands up North were taken by those good-for-naught Elves and Elf-friends.
Maybe that was enough to set up a pattern that generally speaking perpetuated itself over the generations.
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