The second (grey) line are the residue of the pencil markings I erased. I still press too hard on the pencil when sketching. It's an old habit of mine that i'm working on breaking. I recently began adopted holding my pencils as I do with the dip pens and it does work. But will take some getting used to.

I created those two "doodles" with a dip pen so the line widths vary naturally.