What do the children notice about Calpurnia's behaviour in her church community in To Kill a Mockingbird?

Although Calpurnia is well-educated--she taught Scout how
to write cursive--and speaks very proper English in the Finch household, Scout notices
that her language changes when she's at church among her fellow African-Americans. Scout
suddenly realized that Calpurnia led "a modest double life," existing as the Finch
housekeeper but going home each night into her own black
world.

"Cal,"
I asked, "why do you talk nigger-talk to the--to your folks when you know it's
not right?" 

Cal explained
that the way she spoke in the Finch house would be out of place among her black friends,
just as it would if Jem and Scout "talked colored-folks' talk at home." She said it
would be unladylike because it would sound to others like she knew more than they
did.

"They'd think I was puttin' on airs to beat
Moses."