Why is Aunt Polly so angry with Tom after she talks with Sereny Harper about Tom's dream?

Immediately after returning home, we see that Tom is back in his regular routine despite his attempt to change. He easily manipulates Aunt Polly by telling her about the dream. Arriving home in a horrible mood, Tom is confronted by Aunt Polly, who has learned that Tom has lied to her about his "dream." After returning from the Harper household, she learns that Tom had returned home that Wednesday night. Ridden with guilt, Tom apologizes, but