I don't think those seat pickers cannot be trusted for this information. I had one American Airlines flight that showed so full that I had to pay for seats so that my wife and I could sit together. When we got on the plane, it was clearly under capacity and there were plenty of seats together. I complained to the flight attendant a bit just to vent, and later I saw a refund for the up charge. I assume it was due to my complaint, but who knows maybe they have some sort of process that automatically refunds them on under capacity trips.