I'm terrible at explaining things so I'll kind of show a picture drawing of it.
[link]With due of my terrible explanation, the purple lines are like the "path of light" as I would call it so to speak. However the actual "path of light" is much like the path of light in this picture. It makes the highlighting fall kind of flat, but also due to the fact that some of the shading follows a slightly similar path, but instead of falling "flat" like the rest of the lightning, it seems to kind of have a different source of light, causing the shadow underneath the bang on the left.
I hope I don't sound too confusing. The picture isn't bad at all by any means, just something I noticed.