It is the dihedral effect of sweep.

With a straight wing the low wing position requires a fair amount of dihedral for normal stability, midwing just a bit, high wing hardly any or flat. If you then add a bunch of wing sweep the dihedral needs to be reduced the same amount from that base value for all three positions. A high mounted wing will need quite a lot of anhedal to compensate for a 30 or 40 degree sweep which is a common sweep amount for a large high-subsonic airliner.