I suspect that you are correct in your analysis.
I have a WM bag and I understand what you are saying. Yes I agree the higher rated down should have more "springyness" as you describe. Why is it that that the WMbags can be easily identified by feel?
I think my prejudice is against the UL 900 down jackets with 4 ounces or less of down. My old 550 down jacket from REI has been around for 25 years and cost me $60. I think its about as lofty as when I got it. It doesn't compress a real lot while wearing it, I think because is has less compressible of down and probably a lot more down since it weighs 25 ounces.
I believe both of these two concepts to be real fact, how they relate to each other is a puzzlement. There seems to be a dichotomy - the higher rated down should be "springyer" yet it compresses more easily, or is that a misunderstanding? If we use the spring alanogy the 900 down would be longer weaker springs wouldn't it? F=Kx and all that. I'm not certain how "power of expansion" and "ease of compression" can be described in terms of pysics. Also the higher rated down has a higher content of "warm" stuff. My "700" down WM bag is drastically warmer, softer and more compressable than 650 down seemed at the time I bought it.