It's a bug that sometimes occurs when you assign a new aircraft with different a different configuration/capacity. Usually reseting or hitting save fixes things up.
To be honest I have no problem with how people run their airlines, if they want to be a spamline then that's ok.
The game isn't realistic, the Airline industry is far to complex to be implemented into a single game. I like it the way it is and the freedom it provides, even though the AE passengers seem to lack common sense for the most part...