100 watts @1.0% distortion driving a 4ohm nominal load @clipping with one amp.
100 watts @.02% distortion driving the same load using two amps that are nowhere near clipping.
Just an example. But, I can clearly get the idea as to how all watts are not created equal. Mathematically both are 100 watts that should yield equal dB, give or take some added unpleasant noises.