Why? The small cavity of your ears will change substantially in volume, resonances and reflections if the thick cushions compress more. Here is a quick test I just ran on my headphone measurement system. I kept my finger in the same place on the headphone, measured, and then pushed it in some and measured again: We see that the volume clearly changes and the response overall changes in some places. Really, if we are going to consider "burn-in" of the driver, surely there are more changes to the cushion than that element!