It’s a joke. Since everyone claims they were descended from the first group and no one wants to admit being descended from the second group, if you take it at face value it appears that the first group has tons of children and the second group none.

Let's hope it doesn't mean there has been a large scale "transfer" of children from the second group to the first, as it did in many other instances of this happening.

It seems quite obvious to me: the "casquette girls" volunteered and procreated with abundance while the "correction girls" were forced to the colonies and mostly refrained from procreation. The former found themselves a new home, the latter found this new home to be more of a place of exile.