FlyboyDave,
Cast iron and steel have slightly different coefficients of expansion. That means there is at least some difference. Also when a cylinder and piston of the same material are heated up, the cylinder will have a slightly larger internal diameter than the piston's outside diameter. Unless there was an interference fit.

I think Bax hit the nail on the head, you and other machinists, racers, etc. Build engines to very exact tolerances, in fact I mentioned the AA dragster trick of actually honing (maybe it was lapping?) their rings because the only break in was a short idle period after rebuilding between runs. Manufactured engines cannot afford this precision.