I think really it usually comes down to musicians not even thinking about the fact that they are doing it someone else's way. They tend to sing a song more or less as they heard it. Most of us start to talk by imitating what we have heard and I think that singing happens the same way. You are always most likely to sing something similarly to the way you first heard it (if you can). I think that only the better musicians make a conscious effort to adapt what they have already heard.