I flew with easyjet from Basel-Mulhouse (BSL) to London Gatwick (LGW) in September 2012. The flight took exactly 1 hour. I will fly to London on Ocotber 7th, same route, again with easyjet. This time, the flight will take 1.5 hours.
Why does it suddenly take longer? I checked other dates and times; it always takes 1.5 hours now.
One possibility is that somewhere along the line they noticed the flight was often late, so they bumped the scheduled arrival such as to have a better chance of making it. But they kept the scheduled departure such that they can depart on time if the aircraft is ready.
This makes business sense if a reputation for not being on time will cost them more customers than a longer scheduled duration will.
After all, on most routes low-cost carriers such as Easyjet compete primarily on price, not on convenience to the traveler. Their customers will hardly grumble about the flight taking half an hour more when they book it, but they will complain to all their friends if they arrive later than they had made plans for.
Nobody complains about arriving early, though.
The takeoff and landing slots get assigned to the highest bidders and can be changed around.
Also the routing of the airplane can be altered, either for economical (fuel consumption) or for airtraffic reasons.
