Thats a good point. I am thinking of horror films like "Son of Dracula", were its really Dracula himself and there is no reference to a son at all. Similar to the Zorro serials "Son of Zorro" is really about a very distant descendant. The strangest one is "Zorro's Black Whip" the word Zorro is never mentioned at all! Its about a masked woman called "The Black Whip".
Sometimes back than sequels are what we would now call spinoffs at best. Or were using the name as a marketing hook. So the trend started with the Godfather of using numbers was to show it was direct continuation.