Why does Aunt Polly say she could forgive Tom for a million sins?

Aunt Polly has learned from Mrs. Harper that Tom's dream was a fake and that he came home one night and spied on them. Aunt Polly scolds him for making her look like a fool in front of Mrs. Harper and then asks why he came home but still did nothing to relieve everyone's sorrow. Tom replies that he was going to leave a message for her, but he was afraid it would spoil the surprise, so he left it in his pocket. She sends him back to school and goes to look in the jacket that he wore to Jackson's Island, resolving not to be angry if the message is not there. When she finds it, she breaks down in tears and says, "I could forgive the boy, now, if he'd committed a million sins!"