Not every plane was designed to have the FAA-prescribed weight of Americans and their full baggage allowance in every seat from every airport in all weather for all routings at the cited range. The FAA's standard passenger & baggage weights are higher in winter than for summer; maybe the FAA expects heavier clothing, or knows SAD-induced carbo loading.

The plane doesn't need to be oversold to use the weight and balance exemption for aircraft with fewer than 61 seats. http://elliott.org/the-navigator/a-l...-compensation/

Skeptics - cynics - will assert that AA prioritizes paid cargo over passengers and bags. Maybe. Prove it with a class-action suit. AA has a couple thousand contracted flights a day. If this happens with any frequency you should have little difficulty finding plaintiffs. Conduct discovery: AA has records of every cargo item, passenger VDB/IDB, and aircraft takeoff weight and routing. A suit in a U.S. court is how big boys play.