I found it.
Under promise so that people don’t consciously, or subconciously, ask too much of you. If you over promise you’ll end up stressed most of the time…. Think about your own stress levels when you promise.
And, over deliver so that people end up happier than they thought they would. People love surprises, especially the good kind…. Think about their happiness when you deliver.
And you never want to promise or deliver exactly what you said you would. It leaves you looking bland and boring.