If a release has a pregap track and you also activate the data tracks in the editor, the data track will start with a number one higher than expected.

Try e.g. http://musicbrainz.org/release/06c015bb-b3bb-4904-a339-e2b55ea3d6bf/:

0: Pregap
1: Audio 1
2: Audio 2

If you add a data track that will have number 4 (obviously it counts all the existing tracks and adds 1). Expected is 3.