The fact is that your ears/brain DO adjust, even over a very short period of time. If you listen to music in your car, you probably experience this phenomenon on a regular basis.
Do you ever get into your car in the morning, turn the key in the ignition, and then get painfully blasted by the stereo that you left on at a high volume? Do you remember it being that painfully loud when you turned off the car yesterday? Probably not. When it happens to me, it's always shocking how harsh the music sounds at that volume.
Another example of this phenomenon is when you listen to music through headphones on mass transit -- planes, trains, busses, etc. Your ears adjust to the high level of ambient noise, so when you want to listen to your music, you need to compensate by turning up the volume of your music player. If you use that same setting in a quiet environment, it's going to sound loud and possibly uncomfortable.
