It never seemed to me the difference could be only due to hours of practice. An obvious way to interpret this result is that something like 10,000 hrs is a threshold for any violinist to reach their potential. Once you've crossed that threshold other factors may be the difference in good and great.
I remember reading Flow by Mihaly Csikszentmihalyi. About reaching peak experiences which he named experiencing Flow. I seem to recall in that book he found it took 3-500 hrs to learn something, around 3-10,000 to master it depending upon what the activity was. He didn't look at before or after age 20, and didn't compare good with best as I recall. Even then it probably isn't correct to say anyone can learn anything to a good level with enough practice time. It does mean if you want to get good at something you can't shortcut putting in the time. And you have to put in the time to experience flow with an activity.
I know I've tried to learn musical instruments, and it just isn't in me. I was told by so many people just stick with it, you have to put in your time and it will happen as sure as night turns into day. Well the last time I kept a workbook and kept up with the time I was putting in. After 500 hrs I stunk it up like an untalented 5 year old. Other things I've been able to be pretty good at I've estimated hours spent on it, and been astonished it was so many. So no surprise I was okay at those things.
As for musical instruments, I'm a virtuoso in playing the stereo.
EDIT to add: Also think about how much time 10,000 hrs is. Assuming someone started when they were 5 it takes about 2 hrs every single day until you are 20. There aren't many activities you can put that kind of time into. I've found the 3-500 hr estimate to learn something well seems to be just about right. Mastery takes much more.
