Howdy
Good observation. The other grip frames you are looking at are on replicas of the original Colt Bisley model. In 1894 Colt designed the Bisley model for the American team to use in the international shooting competition at the Bisley Range outside of London England. The Bisley Model was a specially setup Colt Single Action Army. The frame of the revolver was slightly different than the standard SAA, but the humpbacked grip frame, the trigger and the hammer were the main differences. The first Bisley models were setup as target models with a drift adjustable rear sight and a target front sight with removeable blade.
Here is a photo of a Colt Bisley Target Model:
Colt also made the Bisley Model with standard non-adjustable sights:
The idea was that the grip shape was supposed to be more hand filling than the standard SAA 'plow handle' grip. Personally, I have always felt that the Bisley grip was developed for the 19th Century style of target shooting. When held the way we normally shoot a revolver today, with the arm extended and the elbow locked straight, a Bisley model tends to point down slightly at the floor. But 19th Century target shooters often held their revolvers with a slightly bent elbow. When held this way, a Bisley Colt will point straight ahead.
The Ruger version of the Bisley model is not the same as the original Colt Bisley, as you have observed. Personally, I have always thought the Ruger version was based on the design that Elmer Keith came up with when he designed his famous 'Number 5' revolver. This photo is not Elmer's revolver, I have not been able to find one. This photo is of the revolvers that Gary Reeder is producing from Ruger Blackhawks that he is modifying to be similar to Keith's Number Five.
As far as why Ruger came up with their version of the Bisley, all I can tell you is it is a little bit longer than their standard grip frame and allows you to more easily cram your entire hand onto the grip than you can with their standard grip frame.