So simple, even a child could understand it! Y'get me?
Just a simple demonstration of how lighting works in regards to models. They are lit according to the brushwork directly beneath them. As such, in this map, when you move over the edge enough that you are above the lower (lit) level, you light up despite being in a dark area.
Not usually very applicable or troublesome, but the more you know!
(I probably could have fixed it so the scientists didn't have the fireflies, but who really cares?)