I wonder if it has something to do with the original financing of the aircraft. Rent/lease an aircraft and you'll be paying for it as long as you have it in the fleet. On the other hand if you buy the plane it goes onto your books and over a period of time you write down the value (to nothing eventually). So now the purchased aircraft has almost no value but your're still paying for the leased plane. Even if the operating cost of the older, less efficient, purchased plane is more than the newer leased plane when you add in the cost of the lease it's cheaper to run the purchased plane because there's no lease cost.