I know you know where this is going, but just indulge me. Which shape is darker? Now, put your finger on the line in between them. Here.
Basically, it works by contrast: When we look at something, we perceive its color and shading relative to other things in the area and how we perceive it’s lit. In the picture here, we perceive the scene as three dimensional, with the light source to the upper left (note the shadow on the ground). The upper lozenge is shaded so that we see it as tilted away from us at its top (making the bottom look shaded), and the bottom one tilted the opposite way, so its top is lit.
Human brains. So amazing, yet so easy to be fooled.