I think it started as a nod to the back action side plates which, it should be remembered, were believed by many to represent a stronger gun.
Later, it became a 'trade mark' of some makers: Rigby, Blanch, Lancaster, early H&H Royals, all H&H back action sidelocks.
It then became devalued as a style by the many low and middle quality guns made with this lock shape and most makers moved away from it, Rigby being the notable exception.
But then Rigby always ploughed their own inimitable furrow!
And Lancaster were just plain odd!