They use the same powder you do, out of the same factory.
All that business about factory load powder only applies to "burning rate" and pressure-velocity relationships. The chemistry is the same whether it is in the Winchester can or the Winchester ammo box. Just that what you get in the can is of standard specification, the stuff in the shells in the box might be off by 10%. Doesn't matter to the factory, they have the test gear to make it work. It can be carried farther than that. When I first got into handloading in the early 1970s, I read that Hercules graded every lot of Red Dot powder into Red Dot 10, 20, 30, 40, 50, 60, 70, 80, or 90. What you got in the can was Red Dot 30, the other grades went to the ammo companies marked for grade so they didn't have to retest themselves.
The reason the factory load is cleaner is because it is loaded in virgin brass with clean brass-on-jacket friction, and high tension for plenty of bullet pull, and a primer fully seated in a clean new primer pocket. That all goes to improve powder burn. Also, it is toward the high end of the pressure scale because that lets them use a lighter charge of a faster burning powder. Cheaper for them, more complete burnup of the load to leave your gun and empties looking nicer.