We've seen an 11% difference in BC between lots in one case. One lot was made with an old worn die, and the new lot (higher BC) was made with a new die. As dies wear, the resulting bullet shapes change, and BCs change with them.
We've also measured a Nosler E-Tip BC at 0.324 that was advertised at 0.469. (150 grain .308). Now I doubt that bullet would measure 0.469 even when perfectly formed, but there seemed to be a poorly formed mismatch between the copper body and the tip. I doubt anyone would have sent those out knowing the BCs would be measured.
It's very simple for a bullet maker to do a few extra quality control checks before sending bullets out if they know the outcome of the testing is likely to be published.
The time or two a bullet maker knew we would be testing their bullets, we got "special attention". In one case, the maker sent a second box of bullets a few weeks later with a note to "please test these instead" because they discovered a quality control problem with the lot of bullets in the first box.
