Because Bill has spent time working on the right side of the painting. The side he moves to now has dried a bit – just enough to pull the paint off the palette knife. Notice, particularly, at the :58 second mark how he just lightly brushes the canvas and lets the paint be pulled off the knife. Bill is barely holding onto the palette knife. It takes a very light touch for this to work.
Also notice how the red he added to the white helps make the highlights sparkle. As we have said before, Bill saw these scenes many times while walking in the mountains where he lived. In his mind, those images were as clear as photographs.