Generally this seems to stem from three causes. The first may simply be an overly optimistic cost estimate. Occasionally under-estimating is politically motivated to ensure project approval. Secondly, any schedule delays inevitably translate into added costs for someone. Thirdly, because most people have difficulty in fully understanding plans and specifications, when the sponsor or the users see the physical results of the project they then seek changes which cost significantly more to implement than they would have, had they been made at the planning stage.