As I understand (correct me if im wrong) the difference between different fill down is how packable it is. For instance, 16oz of 800, 850 or 900 fill down in a TQ, in theory (or at least in my head ) should all have the same temp rateing. But the 900 will just pack smaller then the 800, correct? The reason I ask this is BC with WBGuy's introduction of his Black Mamba, I compared other winter TQ's with the same amount of fill. I noticed that different vendors rated their quilts differently even though their was the same amount of down fill in each. So why is it that quilts with the same amount of fill have different temp ratings? Is it BC of material, construction, or just BC different vendors rate their quilts differently depending on what UQ they imagine people using? Just wondering...a pondering question.