Airlines leasing much of their fleet often split between two different types serving the same segment, like 737+A320 series since the leasing market for two different aircrafts does not change the same way, meaning that if there are few A320s available, the airline could look into the market for 737, knowing that they can operate both.
Having written so, I think SAS can be quite more efficient by standardising the fleet. A few years ago they were serving Scandinavia/Europe with the A321, the A319, B737-400, -500,- 600, -700 and -800, MD81/82, MD87, MD90, Fokker50 and Dash8-Q400. Quite a varity. But, for a plane spotter it was great!