I Wonder What That Means?

I was reading A Wrinkle In Time to my daughter yesterday, and noticed that Meg's mother is "Mrs. Murry" while the witches are "Mrs Whatsit" and so forth. I.e., her mom gets a period at the end of "Mrs." and the witches don't.

My best guess is that this is meant to indicate that "Mrs" is a title for Mrs. Murry, but actually their name for the witches...