Memory and the paradigmatic structures of traditional music have a lot to do with it. I used to play along with somebody who did the original version of Sweetness of Mary. I could never quite get it straight in my head (and he never managed to write down what he was doing in intelligible notation). I now do the Jerry Holland version, which makes a lot more sense to me (though it's not a tune I like very much).

But doing it in A is difficult on most of the instruments I play - G is a lot more sensible.