Quick, let's give it more gear relevancy. Have you noticed how the oof highlights at f/5 are noticeably polygonal? Does the Mk II 500 have a more nearly circular diaphragm?
I'm a bit surprised that the background is not more creamy. Photos I've taken with my 500 f/4 IS show a beautiful, creamy background. Nice photo with nice colors.
Richard - It is probably because the distance between the BG and subject is pretty close as was I between the subject and shooting distance. I have quite a few on N&W forum with better BG's