I've seen many cases in which the same mix/master was used but the files were not completely cancelling each other. If the "difference" between the two files is at -60/-70dB, then it's clear that the files are the same and some filters make the difference. In this particular case, the quad mix is the lossy files from the DVD, 100%, which is fine, but it's good to know where this quad comes from.Why does it not completely null?
Well, it is at least a trip back to the original transfer from the quad analog to digital, and not a rip from that DVD. This would explain the better cleaner sound quality.
For those who don't believe this, just rip the disc and check yourself in Audacity.