Planning fallacy is stranger than it sounds.

The planning fallacy is a phenomenon in which predictions about how much time will be needed to complete a future task display an optimism bias and underestimate the time needed.

The planning fallacy is a phenomenon in which predictions about how much time will be needed to complete a future task display an optimism bias and underestimate the time needed. This phenomenon sometimes occurs regardless of the individual's knowledge that past tasks of a similar nature have taken longer to complete than generally planned. The bias affects predictions only about one's own tasks; when outside observers predict task completion times, they tend to exhibit a pessimistic bias, overestimating the time needed. The planning fallacy involves estimates of task completion time that are more optimistic than the time it took for similar projects in the past.