Why is calpurnia's behavior different in the finch home versus at church?

Calpurnia is the only African-American in Maycomb County that can read. Calpurnia speaks differently in her church because it would frustrate the people there if she spoke the way she does among white people - members would think she is trying to act better than them, so instead Calpurnia speaks eloquently & properly at the Finch home, but at her church she speaks the same way the churchgoers do so as to be accepted into both communities.