Succinctly written, practice.
But, I often use the phrase “it’s a function of time spent”. Especially if I’m feeling a little inadequate compared to another.
When I think it’s just about practice, I can feel I’m falling a little short for not having got enough practice in. When I recognise it’s about time spent (the same thing perhaps, in different words) then it’s easier for me to acknowledge why I haven’t gotten all that practice in. Life’s circumstances and one’s priorities might not lend themselves easily to appropriate practice.
Perhaps the available time was spent elsewhere – quite rightly.
“It’s a function of time spent” both challenges you to make choices and recognises where choices weren’t easy.
Recognise how much choice you have.