It's about stats, not head-to-head play against the rival. Each week the teams are not playing against each other, but the rivalry still exists. Also, these come into play with fantasy leagues. You want your player to have a good week while the others have bad weeks. That Ronaldo v Messi you used as an example also doesn't happen on a weekly basis. Maybe a couple of times a year at best, yet they are still rivals. I'm also confused on when you would have sen Ronaldo track back to be competing for the same ball that Messi would have been playing.