Refer to
this article, and
this one about people with Autism having amazing abilities.
I have seen people be able to do amazing things in terms of art, with seemingly very little practise. I don't just claim this from nothing :P
But you are right too, most people can't just be born with an ability to do any kind of art. Myself for example. I don't have any real raw talent. In order to improve, I have to tirelessly practise and keep trying new things and finding what works and what doesn't.
I've read those articles and saw documentaries of those people, but nowhere does it state that they were just suddenly capable of doing this. Yes, they have a memory that is photographic, either visual or aural, but the fact that they are able to sketch and play these things still include a lot of practice. The article about the autistic painter even mentions him always having drawn and starting at a young age as a way to express himself. It's commitment, combined with an unique memorization skill, that makes him an artist, he was never 'born' with the ability to sketch that well, he had to practice that. The musician has had tutoring since he was five, and again, his skill is a coping mechanism, it's a passion. These people don't get born with amazing skill, they hone it.