In every marriage included in the tree or report there is one individual who is the descendant of the primary individual, or primogenitor (at the top of the tree). The children and marriage information is always associated with the spouse of the descendant individual.

The reason for this seemingly peculiar rule makes sense once you consider the possibility of multiple spouses. The primary individual may have more than one spouse, and children by each. The spouses boxes will be arranged to the left and right of the primary individual, with their respective marriage dates and with their offspring arrayed beneath. 

If the descendant parent had only one spouse, that spouse will always be in the box on the right, with the marriage date and with the offspring connected below.