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.
If you "overfill" a down chamber, the compression of overfilling increases the stiffness of the down chamber and it pushes back at you more than the same chamber filled with a "Normal" amount of down.
To use your spring analogy, F=-KX where K=spring stiffness and X = displacement. If you start with X=0, the feedback force from gently displacing the down is going to be about Zero (0). If you use cloth to compress the spring to X=1 and then you gently push on the precompressed spring, you find the feedback force to be about K, instead of nearly zero.
So, you say, "Whoa, I'm only going to precompress the spring (down) a liitle bit, say X=0.1" Then instead of nearly Zero for a resisting force you have 0.1K as a restoring force. And 0.1K is greater than 0.0K.
If you want to emphasize down pushback as a tactile sensation, then overfilling the down tubes a little is a good way to get it. Most folks will tell you that if you want to increase the warmth of a sleeping bag, overfilling is a good way to get it.
So, overfilling the down tubes gives 1) warmth and 2) nice pushback feel.