OK, I'm getting comments and phone calls that I have lost my mind and no way could blocks A and B be identical. So here, I have chopped the board in half and slid the 2 boxes to be side by side. I have not fussed with it in any other way! The 2 boxes have the same gray value. It's the context that is throwing you off!
Seems like there is a lesson in there regarding the importance of context.