The airline will also cancel or delay a perfectly scheduled on time flight at times. A very basic example as follows :
Two JQ flights sit on the tarmac in AVV. Both servicable. Time is 2000 and one flight is headed to SYD and one to BNE with the same departure time. Pax are yet to board. ( I would say 'Guests' if it was a DJ flight).The SYD aircraft craps out on the tarmac which creates a curfew dilema. No probs, switch aircraft, SYD pax board what was the BNE aircraft and depart with enough time spare to beat curfew into SYD. The BNE pax are told their aircraft has shat itself ( the pax are normally none the wiser) and they enjoy their 5 hour delay at the exciting AVV and arrive into BNE at 0200 or so not even knowing that their original aircraft in pristine condition has got SYD pax home on time while the poor BNE pax get dudded simply because their home town does not have a curfew and they were given the clapped out SYD plane which is now repaired. At the end of the day the airline has saved itself a lot of money getting those pax back to SYD.
But wait for the bonus, the AVV/BNE flight, (remember this is hypothetical) lets say it was JQ 666 gets cancelled (not really) and is rescheduled to depart AVV 5 hours later as JQ 9666. It has now got a new flight number and actually is on time, not delayed at all, as the new flight number (JQ 9666) actually departs on time at its scheduled time of 2345 !
It's all part of the game !