Here's a few simple examples of why late notice delays can/do occur.
Missed approaches - have to fit them back in somewhere.
Whether it was pilot/controller error, weather, or whatever.
That re-sequencing is now an extra 2-3 minute delay for every other inbound aircraft.
Weather/runway changes - losing LAHSO in Melbourne means going from a max arrival rate of 44 to about 24 per hour. This may be caused by a momentary increase in xwind above 20knots. Do the math on the flow on effect.
Preceding traffic considers a high speed descent is 230knots at 40 miles.
Priority traffic. We don't get 7 hours notice of medevac flights, so if one pops up at an inconvenient time, sorry, but you're no.2.
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