Scout notices that Calpurnia speaks differently when she is at her own church with her friends and family. At the Finch house, Calpurnia speaks with proper English. Why does she speak differently in her own neighborhood? In what way is Calpurnia differ

From the text:

That Calpurnia led a modest double life never dawned on me. The idea that she had a separate existence outside our household was a novel one, to say nothing of her having command of two languages. “Cal,” I asked, “why do you talk niggertalk to the—to your folks when you know it’s not right?”

“Well, in the first place I’m black—”

“That doesn’t mean you hafta talk that way when you know better,” said Jem.

Calpurnia tilted her hat and scratched her head, then pressed her hat down carefully over her ears. “It’s right hard to say,” she said. “Suppose you and Scout talked colored-folks’ talk at home it’d be out of place, wouldn’t it? Now what if I talked white-folks’ talk at church, and with my neighbors? They’d think I was puttin‘ on airs to beat Moses.”