Yep. Another factor is whether the black fill was done with different levels of intensity. i.e. - if they pressed down a bit harder with the sharpie in some areas more than others, or if they did multiple layers of ink in one section, but not in others. IME, the camera seems to be sensitive enough to pick up on these subtle differences in shading.
I experienced this with one of my first projects on the PRU, and one or twice since then. It’s easy enough to work around, but yeah - it’s a thing.