I'm sure that the disc manufacturers would love for their products to be more consistent. Then Discmania and Mint Discs wouldn't be touting about their different runs and colors that make the flight patterns unique, but could brag about how one can buy a mold and know exactly how it will behave.
I visualize the manufacturers getting in large shipments of plastic pellets that come from recycling centers. The composition of the pellets will never be consistent. Some shipments of pellets might contain plastic primarily from ground up water bottles, while other shipments contain plastic mostly from plastic shopping bags, perhaps. It is possible to buy plastic that is very consistent, but it will cost more. I expect that the disc companies are buying the plastic at the cheapest price available. And, consistency is sacrificed for lower costs.