Yeah, defensive gun use (DGU) is difficult to quantify.
If the armed citizen is busy, or has some other reason to not want to spend hours being questioned by cops, and nobody got hurt, the "good guy" is likely to just walk away after scaring-off a bad guy.
Some otherwise lawful uses of guns in self-defense are done by convicted felons (not yet pardoned), fugitives, habitual drug users, etc. Not all people who carry guns for self-protection have permits or could legally obtain the State's permission to do so, but they still carry anyway, and sometimes an even worse criminal who means to rob them or kidnap or rape them on a particular day will end up shot full of holes. The shooter may not call 911, if she can just wipe her prints off the gun (one that she bought off the black market and is probably stolen) and toss it in a ditch. With a criminal record for drugs and check forgery and multiple arrests for prostitution, she doesn't want to talk to police. But she's glad she had that Jimenez Arms .32 with her that day, and she'll eventually acquire another cheap gun to carry as she walks the streets.
How can you estimate how many times a would-be home invader pounds on a door, or starts kicking it, but hears a gun being cocked on the other side of that door, and a gruff voice warning him away? That home occupant may himself have some contraband at home, and would not welcome the cops snooping around. In fact, it might be that the reason a home invader targeted his home is because he's known to gamble in the back room of the local bar (illegal gambling) and collect big sums of cash when he wins.
Think of all the illegally armed citizens in states that don't allow mere peons to carry handguns-- states like NY, NJ, MA, and the like. If one of those people pull an unregistered pistol to chase off a car thief, is that citizen likely to call the cops? Not hardly. If nobody fired a shot, they're just going to keep the government out of it.