If you look at Appendix B: Tag Mapping — MusicBrainz Picard v2.11.0rc1 documentation you’ll see that ID3(v2) (and APE(v2) and iTunes MP4) tags, in fact, only have one value for storing both the current disk and the number of total disks: TPOS. So ID3 tagged files do not have a way to separate out the two natively, which is why both values get stored in the same tag.
There was some more discussion about this (though for track numbers) in another topic: