If I place on the piano a grey card and read it with a spot reflected meter, which only reads light reflected from the card (that is, supposing the card "fills" the reading angle of the spot lightmeter), my understand of physics tells me that my spot meter will give me a different exposure than the incident light meter used near the piano.
I am not convinced that my understanding of physics is right, though. Actually I very much doubt it.
If I take a picture of a lit monument at night, the exposure for the monument isn't the same even if I am far from the monument?
I'm really puzzled.