There's a theory that most people sand-bag on activities as an ego-protecting maneuver. That we are terrified to know, with absolute certainty, that we are just rubbish at a particular task.

I think we'd rather see ourselves as lazy than incompetent. Even though objectively speaking, "lazy" is probably just as damaging a hit to your self image. Because admitting to yourself that you're a shit basketball player isn't necessarily going to dissuade you from learning piano, but feeling stuck certainly will.