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.