As explained so many times in the forum, and obviously on the manual at Page 11 and Page 30, when GSX is set to "Estimate the passenger Number", it ESTIMATES the number of passengers on board by counting the number of payload station and dividing that by 220 lbs, which is an industry standard of how much a single passenger + baggages weights, in average.
This, of course, it's not supposed to be accurate ( it's call "estimate" for a reason...) and again, it's not GSX's fault, it's a limitation of the payload model used by the simulator, which doesn't specify the KIND of a payload station, it can be a single man, a group of passenger ( first class, second class, etc. ) or some cargo, it's all "payload" to the sim so, when the airplane is also carrying Cargo that doesn't belong to the passengers ( Combi configs, for example ), the estimate will be off, resulting an higher number of passengers.
Obviously, GSX being the flexible program that is, the "Estimate Passenger Number" option is an OPTION, which can be disabled so, you can insert the actual number of passengers manually, so it can match whatever number the airplane own system has.
This because there's no such thing as a "standard" variable for the number of passengers, it's something airplane developers simulate internally on their own.
GSX has a documented way for airplane developers to SEND what THEY calculated as passenger number directly to GSX, which is used in several products like FS Labs and Aerosoft Airbus, or the Leonardo MD80s, so in these airplanes you don't have to type the passenger number manually, and it will always correct, even if the "Estimate" option in GSX is enabled, since GSX will TURN OFF its own Estimate if the airplane is providing with a number.
For airplanes not supporting the direct exchange of data, turning off the Estimate option in GSX will allow you to set the correct figure.
GSX Manual, Page 30, the "Estimate passengers number option explanation.
