I'm not sure if I really grasp the question from the posts above. However, there is a difference in the storage of TOTALTRACKS and TOTALDISCS between MP3 with ID3v2 and MP4.
When tagged via iTunes, an MP3 file has a track number TRACK in the form of TRACKNUMBER/TOTALTRACKS and a disc number DISCNUMBER in the form DISCNUMBER/TOTALDISCS. This is how the information is actually stored in the tag frames (the internal tag data). There is no automatic splitting of the totals to their respective fields upon saving the tags.
With MP4, the data is stored in a binary field and is split to TRACK and TOTALTRACKS and DISCNUMBER and TOTALDISCS respectively. This also happens if you enter TRACKNUMBER/TOTALTRACKS to the TRACK field or DISCNUMBER/TOTALDISCS to the DISCNUMBER field.