Fine then...here's a theory.
Conventional wisdom suggests that when the ball is above your feet, the swing plane is flattened and therefore promotes more of a draw. But when a player chokes up on the club sufficiently, their swing doesn't necessarily flatten. I'm guessing that the bottom of their swing arc is still at or near their feet level so, whether they like it or not, they will be catching the ball on the downswing and compressing it, something that poorer golfers have trouble with. Better golfers adjust the clubface accordingly and are more able to hit "hold shots" with the ball above their feet.
Conversely, when the ball is severely below a players feet, it is extremely difficult for them to continue their downswing to some imaginary point below the ball and thus they are catching the ball on the forward part of their swing circle. Couple that with the fact that club is clearly not long enough to reach that point (players can choke up, but they can't extend the club), only players with exceptional balance and flexibility would be able to extend their vertical swing 6 or 7 inches below their feet level. Poorer golfers would also have trouble on downhill lies for the exact same reason but do reasonably well on uphill lies.
I guess what I'm saying, is that the ball may go to the right when its below your feet and left when its above your feet for more than just the steepness or flatness of the swing. Or....I'm way off base and the PGA of America is correct for once.