Because it doesn’t seem to matter currently if you play ranked games or casual games, the general experience tends to be the same. But one has numbers and things to go with it. You still get people playing to win in casual games and you get people dicking around having fun in ranked games, and the ranks don’t necessarily indicate how they play as a team and a whole bunch of other things that make it less than ideal.
You can compete against each other for points in L4D while not actually playing the adversarial/competitive mode.
You can kill each other in Portal 2’s co-op by being a dick with your side of the portal.
They’re the only things I can think of that are a gray area and not pretty well defined.