If you go 360/500/1000/2000/4000, you are gradually removing the sanding lines and giving the ball an overall very smooth surface. However, by going from 360 to 500 then straight to 4000, you still leave deeper, rougher sanding lines on the ball, then smooth off only some of the edges. This kind of finish will perform better in oil, but still grab on the backend and turn. Basically, it combines the best aspects of a smooth finish (sharp backend) with some of the oil-handling capabilites of a rough finish.