Especially in certain fields — music, design, and programming all fit I think — there is a huge amount that many people just seem to get or not.
I have a degree in music, so I've seen a lot while I was in school. Yes, some people can work really hard and eventually get it, but I don't think that's everyone.
Especially when it comes to ear training, I know a lot of people who have tried really, really hard, but can't seem to develop the skills. I've seen this in many people, even with great teachers, resources, and tons of practice. I think some people have a fundamental difference in how their minds understand music.
I think this can be seen in the idea that certain programmers are 10x more productive than others, and that most "programmers" can't really program at all, even after years of school.