It used to mystify me as well, but I finally "got it" after getting an excellent "sharpie" brush setting. A lot of it comes from making swift lines and keeping my hand on "undo" so I can keep re-doing a line until it's right. It's not as annoying as it sounds!
I can't draw in tablet yet. Many guys here in DA can do that. But I don't. I like to feel the pencil on paper yet. Just I color and do lights/shadows in the tablet. I'm amazed of your linearts, very fast and very clean. I can do this only in vectors and sometimes in physical and thin brush (and black ink) on paper.
Also, I didn't know the "Scketchbook Pro", man! I'll looking for it.
These days I use SAI for pretty much anything but the process is kinda similar (when I do drawings at this resolution, though). I'm not sure if I do actually have one chisel brush but I can just make one I guess.
I probably should try filling in the flat colors this way because I tend to do it all by hand, and it's kinda tedious :<