this question was on points of veiw once, they say its because for example>>
say the last scene on corrie ie vera passing away, its a very quiet scene and your brain adjusts to that. then the adverts come on which has music on. well the sound of the music would be louder, so mentally you think the adverts are too loud and its the telly. you can try it yourself.
put a music channel on for say 5 mins or so then turn to a docamentry channel and you will see its the same.