- time to design the feature
- time to implement the feature
- time test the feature
And we come out with an estimate: "2 engineers and 1 tester for a week" (or whatever).
That estimate fails to account for all the other work that needs to be done. We have yet to estimate:
- test infrastructure extensions
- other infrastructure extensions (e.g., do we need to provision a hosted site for centralized client logging?)
- documentation modification or creation
- configuration analysis and best practices development (to answer the questions customers have)
- marketing, presentation, and other collateral updates
Just because the product code has been written, and even if the tests have been done, a feature isn't done yet. Don't forget to plan for - and actually do - all the stuff around the software.