US operators have to provide to FAA in their Op Spec how they are computing weight. So, usually is the US national average weight (180 lbs) + the max allowed carry on weight (50 lbs). I know it could sound much, but this provide enough cushion for take-off performance, lower performance degradation, and a simplified computation for gate check baggage (that could easily miss handled and may hurt operations).
They have internal weight tracker to see how the assumed weight system is deviating from actual performance. Last talk with some people from this department, they are pretty closer to the OEM performance numbers with minimal degradation.