difference between hydro and mechanical rocker shaft diameters

On further reflection, I think what he's getting at is the shafts made for stamped steel rockers were (probably) not ground at all. They were probably 'raw' shafts, kinda like DOM tubing. Only the shafts used on bushed rocker arms were ground. This would have been a cost savings, for sure, in production. So while both shafts are "round", the shafts used on bushed rockers are more rounderer due to being ground.