Probably because that was how the stats were presented. And it is hard to translate into per-capita because we only have a rough estimate of number of gun owners. And as per the graph, the handgun numbers are estimates as well. It is meant to show the general trend the accidents are down while supply is up.
So there are what, .33 handguns per people in America, and 1.5 handguns per gun owners in America. Roughly.
