Your looking at the wrong value. Fill Weight is not a measure of warmth. It's a measure of the weight of the down. Weight does not equal warmth. The difference in fill weight here is likely (as strong bad says) the cut. The cut of the bison is more generous (more space inside) and it has thicker insulation (11" vs 10"), so it's fill weight is slightly higher (the total weight of the down used is greater).
So because the cut is more generous it has a greater surface area, greater surface area means more heat lost. Also there is more air inside the bag that will need heating, so it takes more energy (heat from your body) to heat this air. They've compensated (to some extent at least) by increasing the thickness of the down but it's still isn't as warm as the bag with a smaller surface area (Mummy).
They real meausre of the effectivness of the down is the fill power, these both have a fill power of 850+. So the qulaity of the down is equal.
It's a bit odd that your paying more for essentially less down, but the manufacturing of the mummy style bag is likely more complex and the company has also decided that you should pay extra for the extra warmth rating.
I've also just noticed the the more expensive one has a more complicated internal structure. Acording to the links on the web site the more expensive bag is:
V-Block Baffle w/two (offset) draft tubes & full down collar...etc.
Where as the cheaper one is:
Side Block Baffle w/draft tube and full collar
So the more expensive one is designed to prevent gaps in the loft on the sides.