It's hard enough to estimate accurately when you are building your own project with no one breathing down your neck. But be a contractor, where your estimate becomes your salary, and you'll find that you tend to give longer estimates.

I once gave a 3 days estimate for a client project. 2 days to make sure I build it right and a third for testing. It turned into a 7 weeks project: https://idiallo.com/blog/18000-dollars-static-web-page