Many times it comes down to which you heard first (and most). I had a guy tell me, after seeing "The Buddy Holly Story", how Buddy did a lot of Linda Ronstadt songs...

I ran into an Eva Cassidy fan who almost wanted to fight because I told him "It Doesn't Matter Anymore" was written by Paul Anka in the 50s. He swore Cassidy wrote it. Meanwhile, the lineage of that song is Anka to Holly to Ronstadt to Cassidy so he heard it 4th generation and just didn't know any better. (I think Ronstadt's version was the best of them. Why? I heard that one first. The original uptempo versions seemed too "choppy" to me after hearing her folkier rendition.)