If ANY of the surge in gun sales that have hit in the past years have been new shooters, at a rate faster than "old shooters" die, obviously the number of gun owners is increasing.
And when "old shooters" die, what happens to their guns? They're inherited. And some of the beneficiaries are new gun owners too.
I think the groups polling for this info have to know, if they have any intellectual honesty, that the results do not reflect reality. All that matters is if the people paying for the results like them, and if the methodology has no obvious method to discredit it that would make them look bad.