Perception of sound is a differential sense. If you sit in a very quiet room your sensitivity to noise goes up, similar to how your pupils open when it’s dark.

Noise cancelling AirPods and gentle background sounds may provide a smooth enough waveform to be ignored, while stopping the increase in audio sensitivity, and smooth out “spikes” of surrounding noise.

Perception of sound is not just some cumulative scalar, that view is far too simplistic