Years ago, as an undergrad, I had a professor explain why we kept seeing the confusing pyramid models. I never investigated this explanation further, but seems to make sense. Correct me, anyone, if I'm wrong and you know more about the history of the model.
Anyway, what this professor said boiled down to this: Climax used to mean a different part of the story. He said that while today the climax refers to the moment of greatest tension, which occurs just before the story's end, climax used to refer to what we call the midpoint today, a turning point in the middle of the story, quite literally the point of no return from the protagonist's point of view. As an example he referred to Hamlet's murder of Polonius. Up until that point, Hamlet's efforts have been passive aggressive, half-heartedly acting weird and trying to embarrass people with plays. When he kills Polonius, he can't hide behind that passive aggression any more. He can't go back. Thus, all action leading up to that moment is rising action, rising to that pivotal moment. All action afterward is falling action, all falling from, resulting from that one act.
That was the explanation. As I said, it's totally unresearched on my part and comes from a vague memory of almost ten years ago, so if anyone wants to chime in with more information, I'd appreciate it.