The nightfalls are pulling from a bigger pool of items when you clear, so your chances of getting an exotic weapon are much lower. The main point of this was the raids and to show people where they were missing exotic weapon chances in the raids. Everyone is going to hit the same number of chances with the nightfall and Xur. With regard to the raids it is a 75% increase in exotic weapon drop chances over what the average person does in the raid, which is just running through hard mode.