Assuming a lot of those guns are antiques or display items that are never fired/cannot be fired the rounds per weapon would increase.
Example: I own a vintage 1903 shotgun. it is an 8 gage. I can’t buy ammo for it as far as I know. Not that I want to afford anyway. But, it would still be listed as one of those guns that are owned
I would venture to guess there are many more that were uncounted.. such as fully automatic firearms from before the firearms control act of 68.