Sentimentality
Calpurnia is, for all intents and purposes, the children's mother. However she is not prone to cuddly emotions so when she is teaching Scout to write, it is out of convenience, to keep Scout out of the way, rather than because she wants to spend time with the girl or because she finds it beneficial for Scout to know how to write. The lack of sentimentality is not rude though as Scout understands their relationship of mutual respect.