I could be wrong here, but I don't think it's Google, I think they just repeat whatever the websites say. And it's certainly not just Spotify, lots of other websites say X by Y when what they should say is X recorded by Y. And unless you have a really superior collection of friends and acquaintances, Rossey, I bet that some of them will from time to time refer to a song by Y when they mean a song recorded by Y. I put it down to laziness (not yours, but that of the websites and the fans).