And, lo, the toy-swappers heaved a sigh of relief, confident that Hasbro had done their work for them.
Swappers usually try to make money off the swap, or at least swap in something from older (or unrelated) lines. Swaps from the factory happen sometimes. I'd venture to guess some worker just mixed up their Primes by accident.
This. I don't know why someone would want to swap Sentinel for Optimus. Of course, somebody might have gotten both, hated them, and returned them in the wrong packages.