SWAG.
"...some guns go unregistered."
That's the point.
Weirdly enough it's always the guns used by criminals and terrorists.It looks like they don't follow the law and forget to go to their local police station to register their gun before comitting their attack.
Over here we have a mandatory registration of all firearms but of course guns used by terrorists, or by criminals for robberies are never registered (duh!).
They also don't follow the laws regarding magazine capacit and other crap ...
I bet politicians in favor of gun registration really believe it's protecting the public when in reality it's a joke and even a threat to democracy.