To be honest, I don't really know, it just works. It may be from impacts between the bullets during tumbling, because tumbling a bunch of bullets works a lot better than just a few at a time. I usually do about 100 at a time in a plastic yogurt or sour cream tub. Some guys put them in their vibratory tumblers, but I just shake them by hand for 20-30 seconds.
I can tell you the powder comes off the bullets very easily before baking though. You don't want to touch them with your fingers, or roll them around the screen.