My best in physical matters involving sustained effort - like a race or a project - is the best I can see to do, because that effort depends on my conscious and unconscious decisions about my limits, including how much discomfort I will accept.
My best in social and professional matters is usually the best I am willing to do. When I promise to do my best, I am implicitly, and sometimes unconsciously, reserving the right to decide what that is. If
"my best" meant, literally, my actual best, I would promise it to others
very rarely - and, when I did promise it, I would grow every time.
