I have a theory, but it may be a load of twaddle....
I think the P-38 is a Lightning. A twin boom supercharged prop thing.
If the blades were to rotate the ideal P- factor way, then the prop wash would be forced into the middle of the aircraft. This would cause both prop washes to fight each other, perhaps causing lots of buffet. This also happens to be the area in between the booms where the air can fight quite freely. Perhaps they decided this casued too much buffet on the tailplane so turned the rotations around. Thus pushing the individual prop washes away from the boom area each side.
When you look at something say like the PA34 Senapod, I wonder if this theory is negated because you have the fuselage in the way of the two fighting propwashes for there to be much buffet.
Any takers??? Sounds plausible but could be utter cobblers.