Following on from FlyingOnceMore's explanation. It does affect passenger choice. My main office is 20 miles from BHD, and 100 from DUB (motorway door to door to each).
Last year I went at various stages from LHR-SYD and LHR-SIN, both times in F and with mrs Antichef. I had a choice of where I started, and would park a car at the airport. The APD factor won the decision every time. A short trip to DUB, and then in transit at LHR = no APD. Whereas BHD/BFS to LHR then transit = 2xOuch!!