[JohnFen]

> The central difficulty and huge part of many programming projects is actually the "unknown stuff" that is insanely hard to estimate, be it

This is my observation as well.

I also think that devs tend to consider the central problem they're solving when estimating, rather than the totality of the work that has to be done in order to complete a task. If I'm tasked with adding a new feature, my mind is focused on the business logic of that feature and if you ask me how long it's going to take, I'll tend to answer the time for just that. I'll tend to forget the time required for all the other ancillary work around that (documentation, UI changes, code review, QA, etc.) Omitting that stuff results in wildly optimistic estimates.