Hitting the back of the Ramp with the Skateboard
It has been said before that shaving is about reduction not elimination. So you reduce, reduce, and reduce.
My guess is that a blade going WTG has a choice of slicing the hair short or riding up over the hair if it is not sharp enough (and probably still cutting the hair but not cutting it flush).
A blade going ATG is like the skateboarder hitting the back of the ramp instead of the front. It is a lot harder for the blade to push over and ride up that hair if the blade doesn't cut it. And the natural direction for that blade to be pushed is DOWN harder into the skin. That would make for a harsher shave. :medvl: If the hair is already cropped short by WTG & XTG shaving it is easier for the blade to bump over a hair it can't cut through.