Crewmeal, you may be thinking that the a/c flew to MEL and then diverted to ADL. The decision to divert to ADL (which I would imagine is close to their track from from DXB to MEL) was most likely taken en route with a view of refuelling and continuing on to MEL as soon as conditions permited. Presumably the fog in MEL persisted long enough for the crew to run out of hours. So not really a case of very much extra mileage.