It's not the plys themselves -- it's that they are poking through. There's two polys facing each other there, and a sort of "box" made by them into which no light can get, so it comes out as a fully shadowed area.

thou and I, my friend, can, in the most flunkey world, make, each of us, one non-flunkey, one hero, if we like: that will be two heroes to begin with. (Carlyle)