It makes a lot more sense when you understand two things: 1) Untrained software engineers' unconscious inclination is to estimate the amount of time after whic

It makes a lot more sense when you understand two things: 1) Untrained software engineers' unconscious inclination is to estimate the amount of time after which it's probably time to reassess, rather than complete. 2) The distribution of actual completion is not a normal distribution with respect to estimates of completion, it is log normal: there's a really really long tail.