High foreheads can make a low Norwood look like he is receding more. That is why low NWs get either forehead reductions or hair transplants. Zayn Malik can reach a NW3 and still look better or just as good as a NW2 who has a large forehead.
I've wondered this too. I think it has to do with several variables including but not limited to:
Size of the head
Width of the head front on
Size of forehead
Shape of widows peak
How much temple points are lost or how big the temple points are on someone's head
Etc