Answer:
The narrator's description of the mother contrasted sharply with the revelation of the mother's secret, revealing her to be someone she presented herself not to be which surprises and shocks the reader, as the mother was practically described as being a saint.
Explanation:
In A Dead Woman's Secret the narrator described the mother as a rigid disciplinarian who instilled unshakable morals in her children, which resulted in the son becoming a magistrate without pity for the weak and the daughter becoming a nun.
This description creates an image of the mother as a virtuous woman in the reader's mind, as also assumed by her son and daughter.
So the surprise is real when the mother is revealed to be a woman who had an affair with a man that was not her husband, the behavior is not in keeping with who she was described to be.
