I'm guessing it's a sample run and the company realized there was a bug in the silicon and told the fab to destroy the rest of the samples. Instead someone at the fab set them aside for the future when they need some raw material to scam people with.

I find it strange that whoever decided to do this kept the correct datecode on the packaging. Presumably they didn't label them as ATMega 328s until the order came in, it would have been trivial to change the datecode to something reasonable for that chip.