I believe that it is a brain thing, not a physical ability. If you think about it, there are times where a simple phrase on the guitar seems impossible to get. But, then you realize that you just played something very similar in another song. It is your mind that says, "this is a different song, so you dont know how to play it yet", even though the truth is, you absolutely do know how. The practice increases your speed and finger memory, and dexterity, but the mind is what tells you what you can or cant do. Lots of times, when I couldn't play something on the guitar, all I had to do was think about it, and correct it in my head, and it became very easy to play. So being born with ability, is probably more being born with a musically dominant portion of your head being used.