In the following screenshot, one can see that the two volumes of the AYBC commentaries on Mark are classified under Mark 1-8 and 8-16 respectively whereas the two WBC volumes are both found under Mark.

IMO, it is simpler to have all volumes under the book they represent to avoid too much proliferation of categories. Whereas many volumes tend to follow broadly accepted divisions (Isaiah would be a good example), the Continental series, for instance, has three volumes on Genesis whereas many others have only two (or one). Its second volume starts with Genesis 12 while the NAC second volume starts at 11:27 and Thru the Bible at 16. Not surprisingly, the result is a mess:

The multi-volumes of DSBS, NAC, WBC and Thru the Bible are all classified under "Genesis"... 