It is the thermal contraction of the disc from the mold when they pop it out after injection, each plastic shrinks a different amount. In fact, it is very sensitive, even color additives can have an effect. Note that the shape of your disc is never the same as the shape of the mold itself, but you could produce a disc in the shape of the mold if you let it cool inside the mold for a long time (like 30 minutes or more), but then the disc might be too over-stable to be usable.
Anyways, shrinking bends the wing down, and X plastic shrinks more than Z or ESP. Usually thermo-polyurethane (TPU) plastics have the least shrinkage and come out more over-stable than other plastics (e.g., Innova Champion, Discraft Z, Lat 64 Opto, Westside MVP, Gateway Evo, Prodigy 400, etc., etc.).
I have a large stack of ESP Cyclones that are simply awesome awesome awesome. Perfect stability, I have a few that are beat enough that they turn over a few degrees, and others that don't turn over at all. I also throw X Cyclones for a large amount of turnover, and rollers.
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)