Besides the usual unknown unknowns, I've also seen this happen with tasks that involve a lot of coordination in the SDLC. Oh the PR went up at at 2pm PST? Coworkers in EST won't review it until tomorrow. Maybe some back and forth = another day until it clears review. Then QA happens. QA is heavily manual and takes a few hours, maybe with some false starts and handholding from engineering. Another day passes. Before you know it the ticket that took an hour of programming has taken a week to reach production.