It's not that it usually is the Scotty Engineering principle. It is more often a hedge against unforeseen complications, because if you give a realistic estimate that would be true for 90% of the tickets, the 10% will come to bite you. So to not get chewed up for providing realistic 90%-true estimates, you give 99.9%-true estimates that are far higher, but with just a .1% chance of being screwed instead of a 10% chance. Which is, at 10 tickets per day, getting chewed up daily vs. once every two weeks or so.