One of the great variables is the manner of practice. Every so-called virtuoso guitar player has spent thousands of hours with their hands on the instrument. But where one such celebrity may have used a 10-hour-per-day practice routine in their teens, another may instead have taken drugs then listened to music and jammed with his like-minded friends at every opportunity. Very different forms of expertise results.

Quality of practice is another factor. It's possible to become very good at doing things the wrong way, then belatedly realise that progress has been unintentionally stunted. And even that is not clear-cut. Sometimes 'the wrong way' is lauded as creative discovery, and other times simply as reduced competence.