I play around planning flights just for the fun of it and it gives me something to do when I am waiting for either my neuro kid to finally go to bed (some days that child only seems to need sleep from 2-3 am and back up at 8-8:30 or even worse to bed at 5 am other days it is crash and sleep for 12-13 hours) or my DH who works nights to get home from work so I can get some sleep.
Trust me flights change on a regular basis even looking at the same general terms.
Sometimes it is the equipment-flight was planned for a CRJ for example and it filled up fast so they put a boeing or airbus in there, slight change in time flight originally was for 6:23 now its for 6:18 or 6:37. Layover was 45 minutes now it's 2.5 hours (happened to us on the return in 2010 not that we minded-note to self do not use sister as a
travel agent who cares if she as a single adult can make that type of connection it is not done with children and elderly), ect.
Then there is snafus getting from where you started to your desination including but not exhaustive
equipment failure noticed on the ground so your plane has to be replaced with different equipment
flight crew from that flight would be over hours they can fly according to regulations before the flight ends so have to wait for another crew
(both of these happened on the SLC to our home airport in 2010)
Medical emergency of a fellow passenger so the flight is diverted to the closest airport. It is not a case of simply medics come on get the passenger off in a stretcher and your up in the air again. There could be fueling needed along with just having to wait for a spot in the airspace and the busier that airport is the longer your going to have to wait for a slot.
Equipment failure in the air and the plane needs to land before it becomes an alert emergency services and have them standing by on the ground situation or even worse a crash landing. This could be something as simple as the bathrooms do not work-yes I have seen that from a plane that had an emergency landing at an airport close to me.
Flights can also be added such as in situations where an Airbus 220 is being used and it is getting full depending on how many searches are for that flight they may add a 2nd plane of the same style or a boeing. A lot of this is going to be determined by what they have a pilot available for the schedule and what that pilot is trained and certified on known as type setting. According to Mentour it is specific as to if a pilot is trained on a boeing 737 he or she can not fly a 757 series without a certifying pilot even as a first officer.