The FAA average standard passenger weights come from the CDC's National Health and Nutrition Examination Survey which is conducted on about 9000 subjects in a clinical setting each year with results published every 4 years typically. Crew weights come from the FAA's Aerospace Medical Certification Statistical Handbook which is a collection of data from active airman certificate holder medical examinations (it gets down to by medical classification too.... so weight of an airline pilot versus a private pilot, in broad terms as example). The weight used also is adjusted to include your carry-on bag/coat/etc. Sometimes on United (and Delta I've heard this) they really are trying to avoid gate checking bags and making ever effort to get carry-ons in the cabin. That's because if it goes in the cabin, it's part of the assumed weight. If it goes down below, then it's another 30lbs to the weight at least on paper.

If an airline thinks that by route due to typical passenger profile or whatever deviates from the standard weights, they can do their own survey and propose something different.