This appears to happen when there's subsecond stuff added by the discid; the track parser apparently truncates down for display (it's displaying something that's on the release as :21 as :20 in the listing). However, I don't know if it also affects track times that would be displayed rounded-down.
To reproduce:
1. http://musicbrainz.org/release/5ac38f64-27ec-3fe0-9c46-727f3537e877/edit
2. Open the track parser
3. Change anything in the title of track 7 (Juggling~acoustic GTR "turbo"starter~) – I just changed the capitalization of the j
4. Parse Tracks
5. Progress to edit note tab (you may need to select recordings in some cases)
You'll see that track 7 (and some others) have their times changed to one second smaller.
This will not happen if you just manually change the time without using the track parser.
the javascript formatTrackLength is working incorrectly, it's adding 0.5 to the duration rather than 500 (i.e. 0.5 seconds).