Most of the time when a child has a guardian appointed it means at least one of their parents is deceased and that the minor child had an interest in that parent’s estate that needed to be protected. If the father died, the surviving widow may not have been appointed the guardian.
But dead parents were not the only reason a guardian may have been appointed for a child. If another relative died and wanted to leave the child property, they may have indicated who they wanted appointed that child’s guardian upon the relative’s death. Sometimes that guardian was not the child’s parent.
The most frequent situation of a child with living parents being appointed a guardian is when a grandparent was not overly fond or trusting of their son-in-law. The grandparent, instead of giving their daughter an inheritance, gives it to the daughter’s children instead and appointing someone else a guardian.