Principles are ways of successfully dealing with reality to get what you want out of life.
Ray Dalio, one of the world’s most successful investors and entrepreneurs, cites principles as his key to success.
Virtually nothing goes according to plan because one doesn't plan for the things that go wrong. I personally assume things will take about one and a half times as long and cost about one and a half times as much because that's what I've typically experienced. How well you and the people working with you manage will determine your expectations.