I have never given much thought to step wedges used for checking monitor calibration and profiles. Yet now I noticed that the grey-colored steps look themselves like little gradients: always lighter on the left side and darker on the right. For example this.
The far left and right sides are always completely black and white, no gradient effect. Any step block inbetween looks like little gradient with lighter left and darker right side.
What is it? Distortion rooted in the LCD technology or some physiological visual effect?