I like your analysis. If x = .1 then the down has a bit more pressure against the fabric to keep it from deforming under slight pressure, such as a rain shell or wind pressure. As an aside I have noticed that down insulators that flap in the wind lose a great deal of their warm and "indoor measured loft". When you say "normal" are you saying "at the maximum limit of expansion vs the available space inside the garment?" I think that's what you are saying and this is specifically my problem with ultralight garments and I'm not sure that "over-stuffing" enough to prevent deformation by slight pressure should be referred to as "overstuffed, but rather "correctly stuffed". I think the industry is playing too much on the numbers and peoples desire for minimum weight to want to do this, it also costs a bit more expensive down, but heaven forbid that they lose sales because their "improved loft" model weighs 1 ounce more.

One big difference between my Wm Kodiak and a Wm Puma which is rated far lower, is one vs two draft tubes. My back was decided cold when sleeping with my back against the zipper. I took the bag back to WM because I felt it didn't hit its rated value because of this and they put more down into the draft tube, a couple of ounces, enough to add a bit of weight, but also a lot of warmth.

As another aside, the bag is huge, designed for a 250 pound 6 foot 6 linebacker and I'm a little guy who likes to be able to stretch out in my sleeping bag, so I inserted 1/8" diameter elastic inside the bag just under the inner liner, above the knees and near my waist. The bag is a lot warmer because of this. It both isolates three area in the bag lowering any internal drafts to zero AND it bunches the bag up giving more loft, AND I can easily stretch in any direction and then the bag always snuggles up to me. This bag is rated at -5 and would be pretty comfortable at -5, but if I pull my big down coat over it, I am actually warm and toasty at -5. The bag has enough "resistance to compression from outside influence" that the coat does not cause its loft to suffer.
These are my own opinions based on wisdom earned through many wrong decisions. Your mileage may vary.