Why Britain used 18-inch aerial and 21-inch ship torpedoes

Why did Britain use 18-inch aerial torpedoes but 21-inch submarine and ship-launched torpedoes?

Eighteen-inch weapons were lighter and easier to drop safely, while ships could exploit the greater range and power of a 21-inch torpedo.

Aircraft had to balance weapon effect against takeoff weight, range, and the violent dynamics of an air drop. The British 18-inch weapon was lighter and easier to control with the tension-wire and tail arrangements intended to make it enter the water correctly. A 21-inch aerial torpedo would hit harder but add weight and make that drop behavior more difficult to manage. Surface ships and submarines did not share those constraints, so their larger tubes could exploit a heavier, longer-ranged weapon.