Mainly because an 800-fill bag requires less down to give you the same insulation, so it weighs less. In a 32-degree bag the difference is negligible, but in bags rated to zero or lower it starts to add up (more on this momentarily). Such a bag will also compress more, taking up less space in your pack. And, as a general rule, a higher-fill down is a better-quality down, with more fully developed down plumules. So it will last longer and withstand more stuffings and unstuffings than a 600-fill bag.
When comparing the Never Summer and Couloir bags, the down is not the only thing that has an effect on the weight and price. The Couloir is clearly a higher-end bag, with better and lighter materials. Bottom line: the Couloir weighs 3 pounds, 6 ounces, the Never Summer 3 pounds, 12 ounces. Not a huge difference, but for weight-conscious campers and climbers perhaps a worthwhile one.
Overall, I'd say a zero-degree bag should be fine for Patagonia, unless you go to quite high elevations, in which case a minus-ten bag might be better.