Why did Jonas's family have a third child, Gabe? Wasn't there only supposed to be one boy and one girl in each family in The Giver?

The guidelines for family units in the community limit set
a limit of one boy and one girl for each family. However, Jonas's father's job was to be
a nurturer for the infants in the community. He received permission to bring Gabe into
their family dwelling for special attention because he was not thriving in the usual
nursery setting. Jonas finds himself enjoying having the infant to observe and help care
for in the home and is concerned when Gabe's condition does not appear to be improving
with acceptable speed. Jonas is unable to accept the impersonal and uncaring manner in
which children are "released" if they do not grow according to the expected norms, and
determines to flee the community with Gabe in order to save his
life.