Why Software Projects Are Regularly Late

Michael Wolfe provides an excellent answer to the question of why software development estimates are regularly off by a factor of 2-3:

Let’s take a hike on the coast from San Francisco to Los Angeles to visit our friends in Newport Beach.

Other answers point out that coming to a task with related experience certainly helps, but just because you’re an experienced hiker doesn’t mean you know how long it’ll take to hike an entirely new trail. Even if you’ve hiked a particular trail before, it doesn’t prepare you for mud, wildlife or broken ankles.