I remember watching a video many years ago from a dice manufacturer--I don't recall which one. He talked about how some dice manufacturers simply pop their dice out of the molds and toss them into the equivalent of a rock polishing machine and let it grind away with grit and then slather ink into the wells. He took a set of AD&D dice from a cheap manufacturer (you know, 1d4, 1d6, 1d8, 2d10, 1d12, 1d20) and stacked them on top of each other: stacked one way the set of dice rose to a certain height; stacked on different faces they rose a whole dice-worth taller.
Non-uniformity will throw off how a die rolls: if the thickness of the die is different depending on which two faces you measure across, if the square area of the various die faces vary, if different corners have different curvatures, if the density of the plastic across the interior of the die is non-uniform, if the material removed in making the wells for the digits is large--all of these things have minor impacts on the fairness of a die. Sure, these things might only account for a 1 to 2% difference, but many gamblers and casinos make their living off from such margins.
So again, I no longer worry about it: I have precision dice.
(And I've now taken this thread far away from it's original topic: apologies.)