The geometric relationship between push rod and valve at half lift is that they are square to each other. The rocker tip on the valve face travels in two directions during the lift cycle, first toward and past center then back, and the reverse when closing. If you shave the head the push rod essentially becomes longer and it jacks up the back of the rocker, this moves the starting point for the rocker closer to center and what happens is the rocker ends up sweeping to close to the back edge (the rocker side) of the valve at full lift, side loading the valve (causing excess guide wear). Shorten the push rod and you correct that problem.