Why does Calpurnia act and speak differently when she is around her friends?

In Chapter 12, Calpurnia takes the children to First Purchase African M.E. for Sunday service. Throughout the service, Jem and Scout notice that Calpurnia is speaking differently than she does at home. After Cal explains that she taught Zeebo to read using Blackstone's Commentaries, Jem asks her why she talks informally like the rest of the colored folks at the church when she obviously knows correct English. Scout immediately becomes aware of Calpurnia's "modest double life" and asks her the same question. Calpurnia responds by saying, "Well, in the first place I'm black---" (Lee 77). She proceeds to tell the children that it would be out of place for her to speak like white folks around black people. Cal says that if she spoke properly around her community members, they would think she was "puttin' on airs to beat Moses." When Scout mentions that Cal knows better than to talk incorrectly, Cal says that it's not necessary to tell all you know, and that it's not ladylike because people don't like being around somebody who knows more than they do. Cal chooses to speak informally to fit in with her community members and not come off as being pretentious.