apparently it's a compression thing
If you were to try to complain, and record sound levels to compare film with advert, you would find the max amplitue to be the same, but adverts see no need for subtlty or dynamics, so every frequency is at max amplitude. This gives a greater 'presence' for the same dB level.

Not actually tested this yet though, so they may just crank the volume up.
( TheAlmightyBeev, Tue 6 May 2008, 10:17, Reply)