Because in the case of the Rhino, the visible hammer completes the movement separately from the internal cam which fires the weapon. In Destiny, the animations imply that the hand cannons (with the exception of The Last Word) are all double-action revolvers. This is shown when you lightly depress the trigger while playing, you'll see the hammer pull back and the cylinder turn while the weapon doesn't actually fire until you very nearly completely depress the trigger. During this action, the hammer strikes whatever it strikes to fire the gun in the game. The Last Word appears to function as a single-action, in that it must be manually cocked, which allows for the unique nature of the weapon's firing mode. Rhinos can function as both, but in the real-world case of the Rhino, the hammer always completes the action BEFORE the weapon is fired internally. This video explains it better than I can. https://youtu.be/6gQiXoiMl4c?t=58