DOING IS NORMALLY DISTRIBUTED, LEARNING IS LOG-NORMAL

Summary from the TLDR newsletter

A process taking two, three, or five times as long as estimated is much more common when learning is involved. It is almost never the case in software development that you know all or even most of the technical hurdles that you will face in the process of development. This throws everything we know about traditional project management into disarray. Knowing your tooling and having enough experience can help you provide better estimates.