I know they do that but they match up certain teams to face each other every year as well. It just use to be more obvious before the Astros moved to the AL. They would promote them like the Subway Series. Now there is an intrerleague series always taking place.
With the Expos gone Toronto didn't have a natural rival with the National League. So we have just played the Braves and Phillies every year. The 2 teams we faced in the world series.