AndyGFLees there are many reasons why flight times change.
Airport congestion is an obvious example and should be taken as signal that the airline is planning ahead.
Seasons: For example DOH-HKG has a flight time of just over 6 hours to 9 hours. (These are the shortest and longest flight times I have experienced W-E on this route. The longer times in the winter.).
Flight direction E-W flight times are always longer than W-E flights.
Seasonal weather patterns such as SW and NE monsoons.
Bad weather. I doubt any airline would try to fly into or across a Typhoon.
Changes to flight corridors en-route.
The flight route.
Fuel saving by the airline.
Etc. etc.