When Bob Ross scrapes his knife across a canvas to create a jagged rock field, he doesn't have a detailed plan for exactly which rocks will appear and where. But he knows that that it will create a a rough patchy area alternating between two colors that will look like shadows in the context where he's doing it.

When you use the pencil tool across the screen in photoshop, it works just as a pencil on paper.

The difference is not so clear as you imply.