I see this too. In my opinion, the seller believes the ammo was stolen from the military, or might have been. The only way to trace it is by lot number. It’s a stretch, but if the military testified that no ammo from lot 12345 was ever released for DCM or other civilian sales, or sold overseas where it might have been resold to a US importer, that could be used to “prove” it was stolen. No lot number equals no tracking.
I also sometimes wonder why so many sellers refuse to divulge the serial numbers on guns they are trying to sell. If the gun is stolen, the only way to track it, generally, is by serial number. If the seller’s gun’s serial number is on a stolen list, the seller will have to give it up. He might also be prosecuted for receiving stolen goods, depending. So, it’s apparently much easier to just hide the serial number. I’ve lost data about Gyrojet firearms being auctioned because of this. Sad.