Dahermit,
I've shot 3000 rounds loaded with Bullseye in a 1911 before a cycle malfunction occurred. The sootiness comes from a mix of graphite and carbon, but that's different from having flakes of unburned powder or other poor burn issues. So, yes, there are fouling peculiarities to different powders. What I said applies "in general", as I stated, but not in every particular in every specific case.
Conjecture is correct, which is why I used "probably", but its not baseless. What the problem stems from is that while there have been some recent advances, powder making processes used for the last eighty years have defied efforts to control resulting powder burn rates tightly, and lots have had sometimes considerable differences in burn rate from one to the next. Canister grade powders, as are sold to handloaders, have their burn rates adjusted by blending with enough of a past bulk lot that was either faster or slower, as needed, to bring the new lot's burn rate closer to the nominal burn rate, as is necessary to keep load manual data valid. But that costs money to do, so manufacturers buy the less expensive bulk powder and rely on pressure guns rather than recipes to adjust their loads to its variations. However, because of that variation, a powder purchased for one group of cartridges sometimes winds up too far from the nominal burn rate to produce desired velocities at safe pressure or practical loading densities, so they repurpose it for cartridges its burn rate is more appropriate for.
A couple of examples: Board member Hummer70, who worked at Aberdeen Proving Grounds as a Test Director, said the worst example he'd seen was a lot of, IIRC, 7.62 powder that was 30% faster than the nominal burn rate and could not be used to load that chambering at all, so it was set aside for something else. Note that the military is pickier than a commercial loader. Their target velocities for rifle ammunition at a given bullet weight are typically 1/3 what SAAMI standards allow in order to keep different weapon sight systems accurate, and they have a gas port pressure window that must be met while both producing that velocity and not exceeding peak pressure limits.
In the 1960's that practice lead to one lot of WC846 for 7.62 NATO Ball being disqualified as too fast, and being set aside for other ammunition. When 5.56 NATO was being developed, it was found that WC846 was too slow for it, so they tried that fast lot and found it was just right and asked Olin to produce more at that faster burn rate, which Olin obliged them with, but changing the designation to WC844 to be clear which lot was what. Today the canister grade versions are sold as BL-C(2) and H335, respectively, which gives you a sense of the error in that fast lot of WC846.
Another example that isn't direct proof but more a strong implication that powder is repurposed the same way in the commercial ammunition industry came from a fellow on another board who has been periodically pulling bullets from Remington Core-Lokt rifle ammunition in the same chambering (either .308 or 30-06, IIRC) and bullet weight over several decades and found various spherical, stick and even a flake powder in the cases at one time or another. I know of no canister grade flake that is sold as slow enough and suitable for medium power rifle cartridges today, so I assume it was an accidental result of bulk lot variation, but I suppose it could have been an experiment. The main point is that the appearance of the powder kept changing in no apparent pattern, so it must have been selected from what they had in stock that was of appropriate burn rate at the time, and no effort was being made to stick to a particular load formulation.
