Good point. So, if we can clearly demonstrate that this is true with OME coils, we really cannot say it is true with ALL coils then can we? The best we can say is it is true with SOME coils that people have used. I know how it sounds, but that doesn't change the facts.
I question whether you can you really demonstrate it with OME coils, or you are maybe observing something else. Because there are others that have demonstrated (and very convincingly so) otherwise with other coils in a very controlled manner, where the ONLY thing that is changing is spring rate.