The only rationale would be that stopping a lens down usually sharpens it up, particularly in the corners. I don't know this lens' specific characteristics though.
well, I wrote this post 2 days ago and not one single soul passing by has answered my question, let alone the guy who actually took the shot ;-)
why would you shoot this sample at f6.3, with an available lens of f2 ? Is it because it's a FF, and you want everything in focus, even at this distance ?