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We have evolved where the illumination comes from above ie Sun and sky. Your experience and memory tell you that a surface facing up is better illuminated than a surface facing down. Hence when presented with two surfaces that are of equal brightness your brain automatically compensates and assesses the upper surface as being darker.
Seeing is not believing!
I have inverted the image. There is still some background effect (thats another story) but note they look more similar in brightness.
Bert
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