This has been discussed here before, but I couldn't find anything with the search. If my memory is correct the problem comes from the down-to-feathers ratio, down being much better at insulating due to its puffy three-dimensional structure. Feathers are needed in a pillow to provide firmness because a pure down pillow would compress too much to be much comfort to your head. A quick google search pulls up pillows as low as 5% down with the highest I found at 50% down. I think there's some sort of regulation as to what minimum percentage of down must be included before a pillow can be labeled as a "down" pillow(perhaps internally enforced within the down industry or through gov't. regulations, I don't know), but I couldn't find any info on that.
The point being, that maybe the down that is in the pillow is 650-700 fill or whatever is advertised, but the down/feather mix will have much lower fill power, much less ability to insulate (for a given weight), and essentially impossible to separate the two without driving yourself mad. I would guess that "fill weight" when used indiscriminantly refers to total weight of down/feathers. However, I'm not sure how the down/feather ratio is calculated (by weight, volume, etc?). Volume would hardly be fair because then a 50/50 ratio would have a very miniscule amount of down. Weight would be my guess. Maybe someone with some knowledge of the down industry can chime in?
disclaimer: I looked at this before buying down from the various sellers, but never actually cut open a pillow to find out! A 50/50 ratio wouldn't cut the fill power in half, because the feathers do have some volume but I would guess that fill power would no longer be a meaningful way to evaluate insulation. Even if feathers provide X amount of volume per ounce, their structure doesn't seem like it would provide similar convection, radiation, etc. insulation properties.