Anyone doing or reviewing designs should be aware of the fact that things aren’t necessarily what they seem.
Did you know that the “visual center” of a square is actually slightly above the actual center? If you put a dot in the true center it’ll look like it’s below center.
This picture is a classic example. I know it is completely unbelievable that “A” and “B” squares are the identical shade. But they are (click on picture to see enlarged).

The moral of the story? Design for how things appear to the eye. There is no absolute reality you are designing for.