To me, a Frisbee is a Frisbee. I grew up with the Wham-O brand, but probably because there weren't any other brands to choose from. That has changed, and worse yet (for Wham-O), they are no longer the top dog. Or even the number 2 dog. And apparently, Wham-O is to blame for their own problems, and it's all because of the additives that they chose use. White Frisbees were traditionally made white by the addition of titanium dioxide. TiO2 is a great white pigment as it has great hiding power and you can add lots of it without it showing signs of yellowing (unlike, say calcium carbonate). But it is expensive and so people are always looking for alternatives.
And Wham-O found an alternative set of additives:
Regardless, the new additive package was not accepted by the players:
"At the time, Titanium Dioxide was getting pretty scarce. So the [Wham-O] people in Mexico, when they were molding the discs, they put in some Aluminum Oxide and other fillers, instead of straight [Titanium Dioxide]. I don’t know how much you know about polymer chemistry, but the other additives were aggressive and they actually caused some degradation of the plastic and loss of performance."Plastic degradation? Now that get's interesting. Which brings us back to the zinc. We don't know what form the zinc was in - elemental (probably not), sulfide (a white pigment, but not the greatest and it is somewhat pricey) or the oxide (another white pigment). Zinc oxide is my guess, as it is photocatalytic under mildly acidic conditions (pH ~5.5) which would lead to the degradation state. While going from 8 % zinc to 10% zinc isn't going to make that big an impact - but that's assuming that the zinc was the oxide in both formulations. What if the initial formulation was zinc sulfide while in the new formulation it was zinc oxide? This analysis can't tell the difference, so it's entirely possible and it fits the limited data.
Sadly, based on this poor analysis, we'll never know but that is my guess: the TiO2 gets the blame, while the ZnO skates free for the killing of the Wham-O Frisbee business. It's a miscarriage of justice.