Bright sun shining on the target "washes out" the black in the direction the sun is shining from. The old High Power shooters used to say "Light's up, sights up!' meaning that as the sun traveled east to west (North-facing range), the edge of the bull would wash out from 2 to 12 to 10 o'clock correspondingly. The zero you put on your gun first thing in the morning would be too low by noon because you could not see the washed out top of the bull. You would hold lower and hit lower if you didn't correct for the changing light conditions. If a cloud suddenly covered the sun, the glare on the target was eliminated, you could see it as it actually was, and you would then be shooting high! This also applies to windage in matches that span several hours. Add to that the effects of mirage that can displaces the target image in the direction that the mirage is running and can disappear if the sun is covered suddenly, and it's a wonder that we can hit the target at all!!! Voodo, it's all voodoo!