Why was Scout surprised by how Calpurnia acted at First Purchase?

Because Scout has never known Calpurnia outside her home and neighborhood, she is amazed at how differently Calpurnia speaks and acts when she takes Jem and her to church in the Quarters. 
After the children and Calpurnia arrive at the First Purchase African M.E. Church in the Quarters, as the African-American section of Maycomb is called, an angry woman approaches Calpurnia. "What you up to, Miss Cal?" she demands. "I wants to know why you bringin' white chillun to n****r church?" "They's my comp'ny," Calpurnia replies. Scout is amazed when she hears Calpurnia speak this way. After the service is over, Scout asks Calpurnia why she has spoken in the manner that she has to the rest of congregation.

"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." (Ch.12)

Calpurnia explains further that the congregation would resent her if she were to speak as though she knows more than they do. Calpurnia tells Scout that people must want to learn themselves; if they do not want to learn, it is better to be quiet or just "talk their language." She later reveals the reason why she speaks a level of English different from those in the Quarters. She did not grow up there as a girl; instead, she lived at Finches' Landing and Miss Buford, who was Miss Maudie's aunt, taught her to read.