The dropping/increase is due to FSL, I think GSX reads a value far lower than my amount and makes boarding lenght accordingly. But it should be related to FSL original dropping/increasing pax. Umberto obviously correct me if I'm wrong. Maybe also your 737 makes something similar
Yes, this was something to be aware of even with the Aerosoft Airbus. If the airplane changes its load in realtime, to simulate a progressive load or cargo/passengers, this will affect how GSX estimate the number of passengers on board, which is obtained by dividing the total load by 220 lbs, which is an industry standard used to estimate the average weight of a passenger + luggage.
The calculation is made when you select either Boarding or Deboarding so, in order to be sure GSX can calculate the passengers number correctly, you should call Boarding/Deboarding when the airplane has finished its own progressive load simulation, so the weight is final.