The other major factor that does not take into consideration is the vast number of illegal, unregistered, or otherwise questionable gun owners, including quite a few who want no one, particularly the government or the snoopy news people to know they own a gun.
Of course, a lot of the illegal owners are in favor of (law abiding citizen) gun control. It makes their intent easier.
Add the millions that are now buying, and you have a rather large percent of the population.
Remember, these polls are designed not to be accurate, but to support whatever agenda the pollster wants to promote.