Audio signals are not always at the same volume level on every media source. In broadcast television, it has been the practice for decades to use higher volume levels during commercials to help attract the attention of the vewer. This is not necessarily done by the broadcaster, but by those who create the commercials. I also have found that DVDs tend to have a somewhat lower volume level than broadcast television. This is not always the case, but there seems to be a fairly consistent 5 to 10 dB difference in volume between DVDs and television broadcasts. I have also noticed some television shows have similarly low volume levels. These differences seem to be consistent, in that episodes of program A are always quieter than episodes of program B. There is no one reason for the volume levels to be different. Engineers record audio tracks below the maximum dynamic range of the media to avoid generating unacceptable distortions. The level they choose varies with engineer to engineer and with company to company. I think more care is taken in mastering DVDs, because they are expected to have a higher audio quality than broadcast video or video tape recordings. Keeping the volume somewhat lower raises the audio headroom for more dynamic passages. Television audio is often compressed, a process that reduces the audio quality, but makes it easier to maintain a higher volume level.

Excellent question. I noticed this when DVD's were replacing VCR tapes. The volume on VCR tapes was always very clear with the dialogue out front. I suspect the surround sound mixing does not temper the background sound. I find this very frustrating when I have to strain to hear the dialogue, or when suddenly inundated by loud volume of non dialogue sound.