I've had to cancel a number of flights between Doha and Jeddah, and there is always a small discrepancy in the amount refunded. Qatar apologised and explained one of the (many) add-ons was not refunded: I believe this was a security related fee for Jeddah.

If you are interested in tracking down the culprit you might look at the fare breakdown you should see on your ticket, or use Matrix to see the composition of the fare