Because the factory lug is not perfectly flat (true), even after bedding it, the rearward thrust under recoil will not be straight backwards. There will be some amount of push left or right. Picture two wedges pressed against each other, though their surfaces may be flat against each other, pressure on one can cause them to slide. If both surfaces are flat and square to each other, they will not slide. Who knows how much it really effects practical accuracy? Depends, I guess on how bad they are to begin with.