The use of particular names, outside the classic royal onomastics, is not so strange.
Due to the high infant mortality rate, often with the choice of the name one wanted "to entrust" the newborn baby to the protection of a particular saint, and particularly styrong was, at that time, the devotion to saints as Joachim (father of the Blessed Virgin Mary) and Christopher.
Not should it be forgotten that, again at that time, newborn babies in the royal or noble families received immediately the
ondoiement, ie a first part of the rite of Baptism, and at that moment they could receive names that were a "good omen" or of custody/entrustment to the protection of a saint; in the subsequent solemn and public ceremony of the rite of Baptism, which could also take place several years later (remember that at the French Court often the full ritual of Baptism was even delayed until adolescence), the child received a canonical name typical of the royal onomastics.
In other cases, the name could be changed at the time of Confirmation (as happened to the children of Catherine de' Medici and explained
here).