Well, it’s a complicated thing to figure out. The difference seems to be how seriously the couple practices their faith. And this apparently applies to religions other than Christianity, too.
Should it surprise us? Does it surprise you? The less regularly a couple attends church, reads their Bible, prays together and separately, the more likely (statistically) they are to end up divorced. If nothing else, married Christians can take this as an encouragement to be diligent in their faith, and unmarried Christians can take it as an encouragement to find a spouse who will help, rather than hinder, devotional faithfulness.