0.15% of all background checks performed. That's a loophole big enough to drive a - wait, I'm still trying to think of what could be driven sideways through a loophole that big.
As the OP suggests, some of those who failed may not have been fully aware that they were prohibited persons. With the proliferation of crimes that are now felonies, along with the increase in suspended sentences for "lesser" crimes, it is possible for someone not very astute about firearms laws to be convicted of a felony, not realize they are now a prohibited person, and get caught by a background check.
But seriously, does anybody think that BGs think they can get a better price at a gun show, what with the recent price situation in response to the recent availability situation? Or are BGs just not comparison shoppers who are also saddled with an impulse-buying affliction that literally drags them off the streets and into gun shows? And if they are going to gun shows to look for private sales so they can use that "gunshow loophole" then why are BGs submitting to background checks in the first place?