Right. So one source saying 4.5 billion passengers, and one saying 800 million unique passengers. Those numbers look at least roughly reasonable in comparison w

Right. So one source saying 4.5 billion passengers, and one saying 800 million unique passengers. Those numbers look at least roughly reasonable in comparison with each other. If the "average" passenger took 5 flights, then that adds up 4 billion passenger-flights. So the fat tail can be caused by the outliers taking dozens of flights.