One of the factors here is that many if not most ammo companies sometimes make brass for each other. This happens for various reasons, but it's usually because one company gets a big order for a certain cartridge and spends so much time making it they don't have time to make other cases.
Handloaders like to generalize about the definitive characteristics of brass from certain companies, but in reality it's hard to be certain that any particular batch was made by the company stamped on the case heads.
