An even wilder guess: the speed of sound varies with air density.

https://en.wikipedia.org/wiki/Speed_of_sound#Altitude_variat...

If they were calibrated assuming a certain distance from the microphone that "hears" what the wearer's ear is hearing and the ear itself, then it's possible a change in air density could position the area of highest constructive interference at the eardrum instead of the intended destructive interference for some frequencies.