Why does Calpurnia speak indifferently at the First Purchase than she does with the children in their home? What Lee illustrating with this switch and Scout's questions about it?

Calpurnia leads a double life. With the Finch kids, she speaks proper English and articulates her ideas in an educated way. At her church she speaks in the vernacular common to Negro's. Calpurnia doesn't want to upset the balance she lives between the two worlds.