“Mary Poppins” by P. L. Travers (post 3): Why is “Poppins” plural?

I don’t know what the author said about her choice of the name “Poppins,” but it is rarely used as a family name in the real world. It looks to me like a description of how the character functions. She pops in at the beginning of the story, and if she pops out at the end of this novel, but pops in again for sequels, then she will be someone with a number of pop ins.

But why isn’t her name “Mary Poppin” (singular), and why does she refuse to give character references from previous jobs? Character references might have revealed that there was more than one person (or personality) going around that looks like her, which would be difficult to explain, especially in England, where there is a much greater belief in ghosts than in literary doubles and multiple personality.