That might explain it. The bore is basically a spiral. The button tracks it as it's pulled through so the rifling doesn't show the issue but chambering does.Not a cut rifled barrel, it's button rifled- far as I know, that button can't do anything but follow the hole and it's gonna press the grooves in equally- but I'm not a barrel manufacturer.
The video seemed to show the grooves consistent the rest of the way up the barrel. I honestly can't think of a way I could cut a chamber that would look like that if I TRIED. Variation of some thousandths in freebore length is poor/non-concentric setup. Going from zero freebore at the neck to whatever on the very next groove is something I've never seen. Only thing I can think of is that the blank was defective- gun drill wandered (probably all the way up the barrel, who knows) around and it should have never made it past that point, every manufacturer checks for straightness of the hole before rifling. JMO YMMV