Sometimes reading the TOC right after inserting a disc into the drive gives wrong results.
My impression is, that this happens on multi-session discs (sometimes).

This was also verified by kepstin:
A disc had 12 tracks with the first read, and then with the second read it hat 11 tracks (recognizing this as multi-session disc):
http://chatlogs.musicbrainz.org/musicbrainz-devel/2013/2013-05/2013-05-06.html#T00-03-14-299011