I thought that was put in as a rule back during the Obama years when Fast and Furious was going on and they were trying to use border violence to justify more gun control. I think they later expanded the coverage. It was supposed to curb straw purchases.
My dealer would have to report if I bought two pistols within a week of each other (or calendar week, I forget). So I could buy 10 pistols from 10 different dealers in a day, but two from the same dealer would get me reported.