What is the difference in the way Calpurnia speaks in the church?

In Harper Lee's To Kill a
Mockingbird, Atticus has to deal with some state business and is away for a
short time. The children stay with Calpurnia, as always, and on this occasion, they
attend church (First Purchase African M. E. Church) with her one Sunday morning. The
children know how to act with respect. At one point, a woman confronts Calpurnia,
unhappy to see white children with among the congregation. When challenged by one of her
own, Calpurnia speaks the language of her black community. Cal is
asked:

"What
you up to, Miss Cal?"

Calpurnia's hands went to our
shoulders and we stopped and looked around: standing in the path behind us was a tall
Negro woman. Her weight was on one leg; she rested her left elbow in the curve of her
hip, pointing at us with upturned
palm...

To the children, the
woman appears to be "seven feet high."

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I felt Calpurnia's hand dig into my shoulder.
"What you want, Lula?" she asked, in tones I had never heard her use. She spoke quietly,
contemptuously.

"I wants to know why you bringin' white
chillun to n***er church."

"They's my comp'ny," said
Calpurnia. Again I thought her voice strange: she was talking like the rest of
them.

And while Lulu
continues her confrontation of Calpurnia, the rest of the congregation seems to be
moving in on them, but Calpurnia smiles. Her son, Zeebo, speaks to Jem, saying, "Mister
Jem...we're mighty glad to have you all here..." And when the children prepare to leave
after the service, Rev. Sykes tells the children that "This church has no better friend
than your daddy."

When the children ask Calpurnia why she
doesn't speak at church the way she speaks with them, she explains that she would feel
out of place speaking in a manner that wasn't familiar to those in her church, of which
she is a member—the biggest difference is that she is only one of four people in the
congregation that can read. So she speaks as those of her community speak when she is
with them, and in a manner more familiar to the children when she is with
them.