Although the physical brightness is constant over the distance from the ambient object, the observer perceives an "undershoot" and "overshoot" in brightness at what is physically a step edge. Thus, at a certain point, the observer perceives a slight decrease in brightness compared to the true physical value. After that point the observer undergoes a slight overshoot in brightness compared to the true physical value. The total effect is one of increased, local, perceived contrast at an edge in brightness.
It is theorised that the Mach band may be responsible for moths' attraction to ambient objects.