It's not meant to upset you. Airlines currently use estimates to determine how much fuel will be required for each flight based on the number of men, women, and children on board. They then top off the tank with a little extra, just to be safe.
But all that extra weight causes the plane to burn more, costing the companies and adding to carbon emissions.
Across the pond, the British-based Fuel Matrix is in talks with airlines to weigh passengers before boarding, in an effort to have more exact calculations.
The idea would be to ask passengers to step on a pressure pad scale either at check-in or during security screenings, with the data confidentially transmitted back to the airline.