Can anyone explain the gradient merging example? If we sum an image in which luminosity is linearly rising from left to right with the image in which the lumino

Can anyone explain the gradient merging example? If we sum an image in which luminosity is linearly rising from left to right with the image in which the luminosity is linearly rising from top to bottom, then the result should have constant luminosity across bottom left - top right diagonal. So we shouldn't see a circle effect in the top left corner, but rather a rotation of first gradient by +45 degrees. Or are the gradients not linear in luminosity? Or are you guys not seeing the circle effect? It does looks quite different on my other monitor, but I can still clearly see a circle instead of topleft-bottomright linear drop in luminosity on the left example.