When the coal burns down the stack clears up. If we had a photo of the engine probably 100 yards away it might have had coal put in the friebox and started it smoking again.
But yes, some firemen are better than others. I have a video of 3 SG L-131's, L132's going up Tennessee pass. The lead and rear engines were putting out volumes of black smoke where as the engine in the middle had a very light smoke. And that was unusual for that class of Rio Grande engines.