Ime, as a junior dev/ops person, there is almost always scope creep and adding padding grants you room to account for the new idea your supervisor/ user thought of when being midway into development. As far as I can tell, my supervisor also assumes my estimates should be padded more because sometimes you might need wait on human i/o for longer than planned (holidays/ sick leave/...).

one thing that I like, that can help, is to add explicit things in the spec that it will NOT do. If you keep this "types" of functionality you can shut down a lot of scope creep: "we need to send an email alert after the job is done." gets answered "we can do that in a future iteration because this says the feature will not include any alerting or notifications, just log to a file and finish".