When Calpurnia comments on Mr. Radley's death how does Scout find her comment surprising?

Scout found her comment surprising because Calpurnia rarely made remarks about white people. From the text:

“There goes the meanest man ever God blew breath into,” murmured Calpurnia, and she spat meditatively into the yard. We looked at her in surprise, for Calpurnia rarely commented on the ways of white people.