Registered is a misnomer, there are actually only a handful of States that require it.
The Federal Background check is not a registration of a firearm.
If you purchase through an FFL dealer, the dealer has to maintain the paperwork for your purchase.
The Feds can keep a running total of firearms purchased, or transferred through an FFL.
Now the NFA items LEGALLY purchased are in a Federal system which can be used as a registration of sorts.
What none of these polls and statistics ever take into consideration are the illegal firearms in possession of U.S. citizens.