Yes, track numbers will change since a song may appear on more than one album, but the album name also changes and we keep track of that. It is no different for track numbers. If a tag is representing album A, it will contain album A's album name and the track number the the song is at ON THAT ALBUM. Then you have to ask, well what is the correct album to use for a song that appears on multiple albums? The first one released, I guess. The space is there for a track number, to not attempt to include it (as well as the year) would be foolish. If someone decides they don't like the track number to be in there tags, I'm sure it could easily be batch removed with one of the many tag editors.