Win Mp3tag > MAC XLD
XLD dev answer:
For FLAC:
mp3tag writes X/Y style tag to DISCNUMBER filed, but this is not a standard for vorbiscomment. Y value should be written to DISCTOTAL or TOTALDISCS field.
For APE/WV:
mp3tag writes discnumber tag to DISCNUMBER filed, but this is not a standard for ape tag. The disc number tag should be written to DISC field.
See http://wiki.musicbrainz.org/MusicBrainz_Picard/Tags/Mapping