Compression/limiting reduces dynamic range. Think of someone shouting and whispering: the shout is significantly louder than the whisper. With compression, the shout is "pushed" down and the whisper is loudened, thus reducing dynamic range. So, in a sense, it's true that the adverts are not technically louder than the programmes they interrupt; if you measured the loudest noise from the programme (let's imagine a gunshot, for example) you would find it to be no quieter or louder than the volume of the adverts. The key difference is that the adverts have been so heavily compressed (up to the limit allowed for broadcast) that every sound within is at maximum volume - there are no quiet bits.