Yeah, it is not the Secret Sauce Factory Powder, or blended lots, or tested lots, it is that the ammo factory buys powder to specification AND PRICE AND DELIVERY. What you get this month may not be the same as what they will load next month.
Instances I recall:
Glen Zediker wrote that when the AMU got in a pallet of factory match .223, they would pull a bullet and look at the powder. Ball powder went to the shorter ranges and rapid fire stages, extruded powder was saved for 600 yards. The same brand and technical specification, but they knew that ammo loaded with extruded powder was likely to be more accurate. (Which I found in my own rifle.)
An American Handgunner article described the police department that got in a new order of ammo, same brand and technical specification, but it had a brilliant muzzle flash that their old lot did not. Sure enough, pulled bullets showed a clearly different powder and anti-flash additives were not in their contract, so the vendor had used what was on hand or what was cheap.
A gunzine writer got one of the then-new 7mm-08 rifles and some Remington factory ammo for it. He thought the bullet nose looked familiar, so he pulled one and cross sectioned it; Hornady. The exposed powder looked like Win 760. Sure enough, the same load of canister 760 gave a Hornady bullet the same velocity. The first run obviously loaded with commodity components, even though from competitors, to get ammo to the testers.
I pulled bullets from a 9mm and a .45 aluminum Blazer. Both powders looked like Bullseye. Both charge weights were in the normal range for reloads with Bullseye. Was the powder Bullseye?