From light de-shrouding, to major port changes, Steve's offers many stages of cylinder head porting in an attempt to maximize cylinder head CFM while maintaining air speed.
Some people believe that more is better, but in reality, engines require a certain amount of air in a given cycle. Too large of a port kills the air speed and makes the port "lame" killing the inertia, harming the atomization of the fuel. Too small of a port limits the total amount of air that can be flowed but typically has high air speed. So the goal is to raise the amount of total CFM, within the requirements of the engine, while keeping the air speed as high as possible.
Older engines can have some pretty major casting shift, as well as casting flaws, so usually greater improvements can be made, however older designs may not be as good as what is available today. In newer engines these issues have been addressed, as well as better factory designs, though sometimes economy comes before performance, leading to a compromise in cylinder head flow.
Another possible factor is that there are a lot of components that have to be made to fit on any particular cylinder head, (i.e. headbolts, spark plugs, pushrods, valve angles, spring pockets, hood clearance) and there must always be compromises to get the best possible outcome of all of these factors. A head with a poor head bolt spread may have a better port design, but may be prone to head gasket failure. A head with more extreme pushrod angles may lead to a better port design at the expense of quicker wearing components. A head with no pushrods (overhead cam) may have straighter ports leading to more mid-top end flow (at the expense of timing component drag). A head with higher angled ports may give the engine a straighter port angle: lessening drag; increasing the possibility of intake charge ramming; at the expense of hood clearance. A curved port may increase mid range power via air swirl, at the expense of top end power. A small port may increase bottom end torque at the expense of top end power. A large port may increase maximum horsepower and peak torque at the expense of bottom end power.