This is a poll done by Pew Research Center, not a registry of guns that the government knows about. Obviously there is still a possibility that people are lying to pollsters, but there are extremely frequent speculations about things like that (e.g. the "Shy Tory Effect") that almost never end up skewing things by more than a few percent.
I think President Trump proved people are lying to pollsters more than ever. Also Americans lie about guns quite a bit. I've even meet some who don't consider handguns as real guns or call them baby guns.