It is rather straightforward. Some monogamous relationships will establish marriage. Sometimes before the four year mark of the relationship. This means that some relationships that fail in the fourth year will also end up in divorce. The date of marriage does not usually coincide with the date of establishing the relationship, however, and thus marriage data is not useful here.

I'm not sure where you find obsession with arranged marriages? The original comment excluded arranged relationships, presumably because people in arranged relationships maintain a relationship for different reasons than those in voluntary relationships. But if you really want to understand his intent, I'm not sure why you're asking me?