The discrepancy is often a combination of two things
Your torrent client only updates with the tracker once every 30 minutes on each torrent, so there's a delay in the stats updating from the get go.
The stats in your tracker will reflect all data transferred since the time the torrent was added, which includes overhead traffic like tracker announces, and peer connections, whereas the stats against the individual torrents in your profile only reflect the amount of actual torrent content data that has been transferred for the current session. Stopping the torrent, then restarting it starts a new session, which resets the amount in your profile against the torrent to zero.
