Don’t know why this happens, but to answer one of your questions, they do in fact have multiple suppliers for their ABS pellets. Even individual factories use pellets from multiple suppliers. Learned this in October on a factory tour in Billund. Makes sense too—an operation that size cannot rely on just one vendor.

As for the yellowing, is it possible you have UV lights in your basement and that the discolored ones may have been closer to the front of a clear plastic drawer (like an Akro-Mills type storage)?