Thermal contraction and stress is the key to all of this business. As the plastic cools during the molding process, it shrinks, but not uniformly, causing deformation such as lifting of the parting line, doming, etc.. Every plastic has a different thermal expansivity as well as variations in rheology, resulting in a variety of different final shapes after the disc cools.
From what I've read on the web, the earliest discs were made with cheap polyethylene- and/or polypropylene-based plastics. These have a thermal expansivity between 1E-4/K and 2E-4/K, which is actually fairly high for a solid. Now they add other rubbery compounds to DX and Pro, but I think these also have a relatively large thermal expansion. Champion plastic, on the other hand, is made with a high proportion of polyurethane-based plastic, which has a smaller thermal expansion coefficient, around 5E-5/K (or roughly 2-4X smaller than DX).
The upper part of the rim is connected to the flight plate, while the lower part of the rim isn't braced. This causes the rim to shrink more on the bottom, than the top, which bends the nose down and lowers the PLH. The greater the thermal expansivity, the more the PLH will be depressed. This is probably why polyurethane-rich plastics produce more over-stable flight characteristics in the final product than high thermal expansivity plastics.
If the entire rim shrinks to a large degree, and the flight plate is not overly stiff, then you can produce a domed top. This effect is exaggerated if the flight plate plastic contracts less than the rim, which is probably usually the case since the flight plate cools more quickly since it is thinner. Dominess is correlated with the amount of upper rim shrinkage, while the PLH is correlated with the relative degree of upper vs lower rim shrinkage, so these are typically 2 different things although one can imagine ways that they might become correlated (but they certainly don't have to be).
The consistency and speed of the molding process also affect things. If you try to mold discs too quickly, they won't cool as much and will tend to have artifacts that slower produced discs do not. And any other compounds added for color, etc., will give rise to differences. So there are many factors at work. If a disc manufacturer wishes to make a consistent disc, then polyurethane-rich blends, slow production and long cooling times, and using the exact same plastic blend, are all necessary.
PS...Check out the bubbles-in-flight-plate blizzards, vs the bubbles-in-rim blizzards for some of the most extreme variety out there...
Drivers: Starlite Wraith (158g), Gummy Champion Leopard (150g), 1st Run Z-Talon (150g)
Mid-Range: Star Classic Roc (146g), R-Pro Roc (157g)
Putt/Approach: Legacy Protege Clozer (158g), Glow DX Aviar (150g)