When a company like FGM is manufacturing multiple different cores at one time, it's not really that surprising that occasionally (or even more than occasionally), one of those cores will end up in the wrong pile and get covered by the wrong cover. For someone like Vice, or Maxfli, or any of the other brands that depend on FGM for 100% of the manufacturing; they have no way of knowing for certain if the core in the ball is what it's supposed to be.
Taylor Made (who I believe is just ordering cores) has an easier time because you can obviously reject one if the outer layer of the core is wrong (of course if they messed up an inner layer they're SOL).
Frankly, I'd be more shocked if this never happened than by it happening. The only real question is how prevalent is it. Do 1 out 12 Maxfili/Vice/etc balls have the wrong core? Is it 1 out of 100? 1 out of 1000?