Is tracknr different than position?

Yes. Very very different.

Tracknr is the specific track number tag. If you emploded the file onto the central, then tracknr is whatever was in the ID3 tag's "track number" field. If you ripped the file on the central, then tracknr is whatever sequence position that track had on the CD it was ripped from.

Position is the track's ordered-playback position within its parent playlist, or a playlist's ordered-playback position within its parent playlist. This controls the order that things are played when you play a playlist with shuffle turned off. Note that it is only relevant to specific user-created playlists and has no meaning in a soup view.