It isn't block growth. Encore flight plans at max cruise to ensure block times are min for a variety of commercial reasons including flexibility, keeping times close to jet speeds on many routes and various other things. Blocks are built with a decent bit of wiggle room for deicing etc. and so flights often push back early if things are going well. If the flight is running early, a cost indexing app is run and it determines the optimum speed to fly given the commercial considerations of the route and that specific day. It tells the crew what speed to fly, and huge savings result since the plane will still make good money at max cruise.You hear the Encore guys amending and slowing down all the time. rarely hear the jazz machines doing it.
Two words. Block Growth.
In addition, crews are advised not to arrive too early or gates may not be ready and that necessitates ground holding which is widely hated by pax so crews may elect to slow down further to meet the windows. Surprisingly, many crews fail to see the advantages for them (block growth) and are reluctant to slow down even when the app tells them to (go fast and stick it to the man)!!
