Most programming can be made modular in a way that many, even all some times, functional requirements can be made distinctly areas of the codebase. It makes sense to break them down to singular subtasks for making various requirements explicit. Add general things such as documentation, deployment etc, and you're able to provide far better estimates.

Through retrospectives teams slowly but surely create a checklist of things to consider when estimating and my experience is that teams get better and better at anticipating the scope of a task and estimating it.