Why does Bob Ewell say he didn't run after Tom Robinson after he caught him raping his daughter?

When questioned, Bob Ewell claimed that he didn't run after Tom Robinson because he was too distracted by Mayella's screaming to do so.

Accordingly, Bob Ewell testified that he saw Tom raping his daughter, Mayella, when he came home. When questioned by Mr. Gilmer, the prosecuting attorney, Bob Ewell stated that he had to drop his load of kindling when he heard his daughter's screams. Supposedly, Bob went to check on Mayella but accidentally ran...

When questioned, Bob Ewell claimed that he didn't run after Tom Robinson because he was too distracted by Mayella's screaming to do so.

Accordingly, Bob Ewell testified that he saw Tom raping his daughter, Mayella, when he came home. When questioned by Mr. Gilmer, the prosecuting attorney, Bob Ewell stated that he had to drop his load of kindling when he heard his daughter's screams. Supposedly, Bob went to check on Mayella but accidentally ran into the fence in the process. After he disentangled himself, Bob Ewell mentioned that he looked in at the window. That's when he apparently saw Tom Robinson raping his daughter.

However, Bob Ewell also testified that he ran round the house in order to get in. Interestingly, he didn't bother to try to stop Tom when he saw Tom run out the front door. Later on, Bob Ewell's story starts to fall apart when Atticus reveals that Bob is left-handed. As Mayella's injuries were sustained on the right side of her face, Atticus was trying to show the court that Bob Ewell could have beaten up his own daughter and blamed it on Tom Robinson. The fact that Tom's left arm is disabled (due to an injury) further solidifies Atticus' skillful portrayal of Bob Ewell as an unreliable witness.