The other thought I had was that C might be the one with the finest nozzle (is that the mircon) and to get the line the same thickness, he had to hold the brush further from the paper to get the line the same thickness as the others. This could explain why the lines aren't as dark? Or that the lines doen't look as smooth as the others?
Either way I get the point. It just means I have to practice a lot more
