Timtam, the reservations systems of ALL airlines world-wide allows a certain percentage of overbooking in every fare category and for every flight and route. Generally first class is only booked to capacity, not over. Coach can be oversold according to the history of the flight and how many passengers have been no-shows in the past. Once the flight departs, the airline has no way to make revenue on those empty seats so the regulating authorities allow the airlines to overbook but with restrictions on how the passengers who are "bumped" will be treated. You can't argue with QR overbooking but sure as all get out can make certain that they treat those who are denied boarding according to published regulations.
Mandi