Different powder... and perhaps an influence by the kind of primer. The main diff in what manufacturers work with is that their powder lots aren't canister grade... so they work up on a lot that's not the exact same characteristics of the powder you'd buy on the shelf.
They don't have magic powders any better than what you or I can get. It's just that they formulate a load on the lot of powder they are running. For example, I've pulled different years of XM193 down and found that you get different weights of ball powder.