The difference between the two is that a government-mandated registration scheme that mandates up-to-date status can clear a lot of legal bars that a private list compiled from random sources does not. Just because someone filled a warranty card for a gun 10 years ago doesn't mean that they still own that - or, indeed, any - gun today, and courts would generally balk at making such an inference for the purposes of, say, signing a warrant for a search.