However the assumptions they've been using for passengers are outdated. Americans are getting fatter, and the federal government wants airlines to find out how much fatter their passengers have gotten, at least for smaller aircraft.
The FAA realizes that passenger weight can vary by route and airlines may want to document this difference.
Standard weights may not be appropriate for smaller planes, with smaller sample size and greater likelihood of variance from average.
Airlines can use standard weights published by the CDC for larger aircraft, with variance for winter and summer based on greater weight assumptions for clothing in the winter. However they outline a method for smaller aircraft to determine "[a]ctual passenger and bag weights" and to determine whether aircraft up to 70 seats should be considered small or large for this exercise.