Developper estimates
Management tries to dertermine a magic number between ideal days and real days. The rational is always the same: PTO, meetings, illness…etc.
But, even if an ideal developper would exists (never ill, no PTO and no meetings) there will always be some shift between estimates and time to developpe the feature. I see at least one reason which has a significant impact IMHO: he is not working alone! He will have to synchronize with the source control before check-in. This can be done efficiently if the SCM system is efficient and longer if not…or if there is no SCM system. The result of this synchronization is absolutely unpredictable.
Maybe, the “single brain” (a team of…1) is the right approach to get accurate estimates…but it has its own drawbacks.
The best is probably experience.
Another step before check-in which may be longer than expected is the refactoring.