In what way does Adam's revelation about his mother's departure surprise Amal in Chapter 25?

Amal is unable to hide her surprise from Adam when he tells her the somewhat anticlimactic circumstances behind his biological mother's departure from his life. In fact, she tells him that he must be lying, causing him to laugh out loud. Adam knows that Amal had been expecting a chaotic story with lots of drama, and he takes some pleasure in her disappointment at his story's apparent banality.