The 10,000-hour rule is arbitrary.
It’s like saying it takes 2 weeks to read a novel cover to cover. Yes, maybe on average it takes 2 weeks for most people to read a novel, but there are several factors that affect how long it takes. Reading speed. Length of the novel. Time spent reading. And many others. Some people might finish a novel in a day, while others take months.
Unlike reading a novel though, you don’t have to finish to achieve mastery. It’s not like with 9,999 hours of practice you’re not a master, but at 10,000 hours you suddenly become a master. Likely, you won’t be able to tell the difference between someone who worked 8,000 hours on something and someone who worked 10,000 hours on it. It’s the difference between someone who is better than 99% of the world and someone better than 98% of the world. They are both world-class.
It also depends on how you train as much as how long you train. Studying 5,000 hours under the eye of a master may be more effective than studying 15,000 hours haphazardly. How else would some athletes be so much better at their sports than others of the same age and similar training schedules? The best athletes are not the ones who trained longer (though that does help), but the ones that trained best.
I’m not saying the 10,000 hour rule is false. But it is just a rule of thumb. For most people, that is about how long it takes to achieve mastery. But recognize that there are shortcuts. And on the flip side, if you do it wrong you might not achieve mastery even after 15,000 hours.