So, I'm looking at purchasing a top quilt, probably the HG Econ Burrow, and probably in either 20 or 30 degrees. Am I wrong in assuming that the amount of down is the primary factor in the temperature rating of a TQ?

As an example, a 30 degree Econ Burrow (standard length/wide width) weighs 20.16 oz. A 20 degree (same dimensions) weighs 24.59 oz. So, is the 20 degree the same dimensions, but with 4.43 oz more down? Granted, cost-wise, the 20 degree is about $20 cheaper than the 30 with 4 oz. overfill, so if that were the case, it makes more sense to just get the 20.

So, if I'm wrong (and that never happens ), and the temp rating isn't a function of the amount of down, what does overfill/stuff actually get you in a TQ? What is the advantage and why does it seem like everyone chooses that option?