They really don’t know.
Lots of WW1, WW2, and Korean War bring backs are floating around the country and I bet a lot of those are off paper and out of the record system.
Many guns that were on paper have ended up destroyed, lost, or stolen so the final disposition of those is a big question.
Some of the junk guns that floated around over the years end up in the landfill. Some bought new, were so worthless and dangerous, think sat nite specials, that owners sent them to the landfill. What else do you do with guns that are unsafe and probably can’t be made safe other than destruction unless you know of a gun buyback program somewhere. They’re junk and should probably be treated as such. There is nothing like losing your face, eyes, or hand because you kept using or playing with a damaged and not repairable POS weapon. What I am saying is that many guns on their list are 100 feet at the bottom of landfills.
I even heard of a story from a friend who was in the military that when he was out of country his mom threw away a 1911 Colt (in the trash), because she didn’t want it in the house after her husband died. It made me sick to hear that. You can bet that has happened many times.
Whatever number they are throwing around is not accurate and the only up to date records are probably of recent purchases, say in the last 25-30 years.