There are two issues at hand: 1) People are pretty terrible at understanding things exponentially (or overall, just non-linearly). Bumps in the road for progra

There are two issues at hand: 1) People are pretty terrible at understanding things exponentially (or overall, just non-linearly). Bumps in the road for programming I've seen can set a time goal back by 10x. Thought it would take a week? Suddenly its more than two months. 2) As highlighted here, we're often bad at translating all of our assumptions and intuitiveness into code. Its just a website with a login, with a, etc.... Yet, there's a lot of details there that you're forgetting about. Experience is the best thing here you can hope for, but even that can fail. Unless you're doing exactly what you've done before, you're going to hit bumps. Also, no client wants to hear an overly long and safe estimate. You might even think it will take 6 months, but the client wants to hear 2. Everyone else is telling them 2. You look incompetent if you say 6, so you say 2... maybe 3.