The people who make the timeline have to make it seem like the completion is sooner than others, deals are made on that assumption. But when building you have so many different workers, and if the foundation can't be laid because of bad weather then you can't have the plumber come in the week after. This plumber then can't reschedule to the desired date so it gets pushed back.

You make a budget but then one of suppliers drops out, so you find a new one that can do the job on such short notice but you pay the price for that. Also You always make the budget as tight as possible so that it is very attractive, but there are clauses so you know you can go over budget in certain ways.