The other thing I was also going to say was that often ads have music and it may simply be that our ears perceive a difference in volume between the spoken words of the programme we've been watching and the sudden break to something unrelated to it with contrasting sounds and music.
i don't think the actual volume goes up, thats not allowed but they do use special filters that make it seem louder. well thats what i read somewhere, can't think where though.