Dynamic range. You could have a soundtrack like many CDs (not all obviously), where everything is overly compressed, and then the quiet stuff would be louder and the loud stuff would be quieter. None of us want that.
If you do a search, you'll also notice quite a few threads about how DTS soundtracks are louder and therefore are perceived as sounding better.
The fact of the matter is that not all DVDs come from the same place. There are no set standards in place saying that a DVD soundtrack has to conform to certain criteria or whatever. Studios and whoever else is involved in transferring a movie soundtrack to DVD all go about it differently. You notice that with different editions of DVDs you get different soundtracks a lot of time. I'm no expert on any of this stuff obviously, but without standards in place, you are going to get varying results (some good, some bad).
My recommendation is before you buy a DVD, do some searching for sites that rate the DVD and its soundtracks (if you care that much) - thedigitalbits.com is good for this, but there are many others.
Also, get used to using the channel level trims on your receiver. Since every movie is different, you may need to adjust things slightly different for every movie if you are picky about the presentation (like me
).
