Our minds alter the world so that we can operate better in it.
Do you see ridges between the strips? In other words, for a given strip, the left side is lighter and the right side darker?
You should, or your brain is broken. Our mind does that to highlight edges, so we know where objects start and stop in the real world.
This one's better:
See the tiles labeled A and B? They're the same color. Really really.
You see tile B as lighter because your brain figures, "Oh, there's a shadow! So if there wasn't a shadow, that would probably be a white tile - like those other white tiles outside the shadow. So let's adjust - and see it as whiter." Which is great in real life; otherwise, shadows would mess you up all the time. But here, it's a trick!
Moral: Your mind enhances the world. And by enhances, I mean lies to you about.