Madeline & Miss Clavell

In the childrens’ series “Madeline”, the character known as “Miss Clavell” (and forgive my spelling) dresses like a nun. Why is she referred to as “Miss Clavell”, rather than “Sister Clavell”? Anybody know?

My sister Kate, who is a kindergarten teacher, was asked this question and couldn’t answer it, so I need the answer to pass on to her.

Wild guess - it’s a bad translation from the French.

Is the old house in Paris that is covered in vines a private school or an orphanage?

Madeline’s father comes to visit her in one of the books.

No clue about the nun-hood or otherwise of Miss Clavell. Although why a nun would be running a boarding school is beyond me. A parochial boarding school, maybe?

Regards,
Shodan

This site provides a lot of information (including the point that Ludwig Bemelmans was an Austrian immigrant to the U.S., and not French at all):
Madeline

Unfortunately, it does not mention why Miss Clavell is not Sister Clavell (that I could find).