Why do i need adjustable pushrods?

Guys, this might sem a stupid question, but bear with me.
when I built my 360 two years back, my standard pushrods were too short. I have used adjustable pushrods ever since and have to set the preload manually. My simple question is why did this situation occur?

Engine spec
360 block-standard height, not decked
Comp Cams XE262 grind. 47 lift.
Smog heads completely rebuilt inc new seats and back cut. Lightly cut/skimmed to true the gasket face
Not a radical set up.
ANy ideas? I would really like to just use fit n, forget pushrods.