Flight plans are filed in advance, but a flight plan is just a plan. It's an impossible task to predict where an airplane will fly in advance, there are so many variables involved.
Just some of the many reasons a flight will vary from flight plan:
-Avoiding convective build up (thunderstorm)
-Avoiding turbulence for passenger comfort (changing altitude)
-Unexpectedly high headwinds (changing altitude)
-A cold front causes winds to shift, so the runways in use change
-Low clouds limits arrival/departure rate, so holding is needed
Just some of the many reasons a flight will vary from flight plan:
-Avoiding convective build up (thunderstorm)
-Avoiding turbulence for passenger comfort (changing altitude)
-Unexpectedly high headwinds (changing altitude)
-A cold front causes winds to shift, so the runways in use change
-Low clouds limits arrival/departure rate, so holding is needed
-Snow removal from runways/taxiways