I will be the first to tell you that I'm not the most perfect rider. However, the second time I trotted a horse I learned how to post without anyone even telling me what it was (It was the only way to save my bum!!), and without any lessons.('what are you doing?' -- 'I have no idea, all I know is that my butt doesn't hurt when I do this!!') I then went on to 'fake it' with very, very good trainers, but they found out quickly that I was 'faking it' and got me to fix it when I got lessons. I am an athletic person in nature (soccer, track), and I think most athletes have little problem learning to ride (as long as they are have some coordination!). They understand balance and the like. I started riding 'late'--around 15--and I consider myself now to be a very decent rider, especially with problem horses.
I'm not saying everyone can fake it, just that athletic people tend to look like 'naturals' in the saddle. If you look at their knees, their heels, or when they canter... you can usually find that they are not as good as they seem.