The thing that makes this non obvious is not just that things take longer than you think, but rather as time goes on you have to adjust your expectation of completion to be longer.

For example, if there is a project that is meant to complete in a week. It has now been two weeks, so one week over budget. Most people would think that finishing the project is right around the corner, but rather the expectation should be that it will take another week or two weeks. If you get to the end of a month, same applies - the expectation should be that it will be another month, not that it is right around the corner.