I have no real idea but it seems that the schedules must have a degree of fat built in so there is some scope for "putting the foot down".
Wrong way round.
If schedules have a buffer built in to them, it's to lessen the need to "put the foot down" following a delayed departure.
Having said that, airlines have to tread a very thin line between being over-generous with scheduled block times (which impacts on utilisation) and trimming them back too much (which affects punctuality stats).