If we're talking about the double-shot legend process, then variability is the result of two things: font choice (or in the case of icons/graphics, the correctness of the vector file sent to SP) and manufacturing tolerances.
SP only has three typeface sets, and 99.9% of their double-shots are made with their Gorton Modified typeface set. That set comes in three styles: normal, condensed, and wide (basically normal with extended Os and Qs and such). Each comes in numerous sizes. Designers will work with SP to come up with the font that will fit best on each key, but if a designer decides he wants condensed Gorton Modified on the SCROLL LOCK key, but wide Gorton Modified on the CAPS LOCK key, that inconsistency isn't SP's fault.
Of course, those fonts aren't digital, they are old-school metal blocks, with all the kerning issues that go with that. When you talk about things designers can't control, the shape and spacing of Gorton Modified letters is definitely one of them, but its the same for all designers, so for instance all SHIFT keys are SHIF T keys; its not like one keyset will have SHIFT while another will have SHIF T.
When it comes to icon/graphic legends, there has been a history of miscommunication/misunderstanding on the part of designers and SP alike. Some designers simply don't know how to send a proper vector file to SP, and SP simply doesn't know how to inform/educate designers so they get it right. Oobly is working diligently to improve this situation. We all stand to benefit from his hard work in this area.
Minute variations due to the nature of the plastic double-shot process are impossible to avoid, at least with the machines SP currently uses. Not much can be done about that without them spending big $$$ on new tooling.
P.S. If it's true that Modern Selectric was designed by SP themselves, then that is one example where we can lay the blame for its poor aesthetics squarely at their feet. But that is an isolated case, is it not?