Most projects take longer than planned. They’re often more complex than anticipated. And there’s usually something that surprises the team, something that goes wrong, or something that was forgotten in planning.
Despite having seen the above play out dozens of times, most teams default to optimism and think, “This time will be different”.
The laws of physics don’t change too often though.
So one way to avoid a headache on complex projects is to deliberately add a multiplier. It could be a complexity multiplier, or a we’re-going-to-discover-new-problems multiplier, or a just-to-be-safe multiplier. You choose.
The multiplier gives a bit of breathing room for things to go wrong, because they almost always do.