If you fill a space with the exact amount of down needed based on it's fill power rating, it will only fill that space well if conditions are perfect. It has to be distributed just right with no moisture and like new loft. As we know, conditions out on the trail are often not perfect. Therefor it is best to have a little overstuff so that if a space opens up inside your sleep system there is a little extra to push into that space. A bag with no overstuff will tend to clump up and shift around, leaving big cold spots. I go with 20% overstuff to keep all the space filled with down. Combine that with vertical split baffles and a differential cut and there is almost no down movement.