This groups songs by their Album Artist and Album field and finds cases where some but not all their songs have been matched to MusicBrainz.
So here we see that in the folder Disc 1 no songs were matched to MusicBrainz, but all songs were matched in Disc 3. So this album did not come up in the incomplete folders section because the individual folders are consistently matching all or no songs, but when grouped by album artist/album we see the issue.
