Reflective metering. The "zones" you see, and place, are the result of both how much light they receive and how much of it they reflect. If you use incident metering two elements of the picture that receive the same light but that you place on two different zones (a black cow and a white cow) will appear to "fall" on the same zone. Reflective metering keeps into account the different placing of the different cows on the film's curve.
With an incident meter under even illumination you only meter one value and have no idea of the degree of "inner contrast" of the scene.