The ripped audio or video from the Deluxe Bonus DVD, or a fan-curated collection of deep cuts that didn't make the official 24-track compilation. Reason B: Multi-Part RAR Splitting
The most charitable explanation is that a user referred to the bonus DVD as a "Volume III" in the filename. The DVD contained no new audio tracks, but a casual uploader might have included it in an .RAR archive labeled "Volume III" to signify the third disc in the deluxe package. However, the official 2007 Deluxe Edition is rarely referred to as "Volume III."