It’s an optical illusion. Probably because the human eye tries to interpret stars/galaxies of different brightness classes as sitting on different layers, and t

It’s an optical illusion. Probably because the human eye tries to interpret stars/galaxies of different brightness classes as sitting on different layers, and the speed perception is influenced by brightness (dimmer stars may slightly lag behind in human perception). It’s likely reinforced by the fact that the speed on the “canvas” isn’t uniform (the dots move faster the farer away they are from the center).