The subject pretty much says it. When sorting by "Album" they are displayed in the following order:
Volume 12: 60s & 70s Pop Classics
Volume 2: 1961-64
Volume 3: The Mid Fifties
etc
(I don't have "Volume 1" yet)
Tried with some other albums, changing the Album tag to "Volume N" and they're displayed the same way:
Volume 13
Volume 15
Volume 8
MB version: 3.3.6962