In addition, there is a maximum limit to the number of aircraft per hour which can occupy the tarmac (both runways and stands). Look at the airline timetables and you will find that the airlines take little account of this when writing their schedules. There may be around 20 or 30 aircraft planned to depart or arrive at the same time which obviously is a physical impossibility in reality. So someone somewhere will either pick up a delay on the ground or in the air.
Actually there's a complex system to try and alleviate this situation. An independent company called Airport Coordination Ltd (ACL) take the number of movements that ATC agree that they can handle in any given hour and then allocate these movements runway 'slots'. These are quite different from airborne slots that are allocated for the en-route part of the flight. Thus at LGW, ATC have agreed that in some hours they can handle 49 movements. ACL then farm out that number of slots for that given hour. The airlines are then supposed to write their schedules around this. Of course, they occasionally can't and we sometimes find that ticketed times are WAY off the slot they've been allocated. With scheduled, well established airlines who have 'grandfather' rights to certain slots, there's no problem, it's usually the newer, charter operators who get in a bind.
We do closely monitor slot performance and constantly try to improve it. Very rarely, we have to deny an aircraft departure approval until their due time. We don't like doing this as often there's a planeload of people on board but by allowing one a/c to 'jump the queue' the knock-on delays can affect many times that number of people on other a/c who are operating within their agreed times.
Of course we and ATC will bend over backwards to help genuine delays or getting a rescue a/c to another airport to put someone's programme back on track.
Cheers,
The Odd One