Why did mayella become angry with atticus when he questioned her?

When Mayella is on the stand the question Atticus asks that makes her furious is "How?" She has just described how Tom Robinson choked and beat her, and Atticus asks Tom to stand up. This draws the jury's attention to the fact that Tom's left arm is useless, making it highly unlikely he was Mayella's attacker. When Tom stands, Atticus asks Mayella if Tom is the man who attacked. When she responds that he is, Atticus asks, "How?"