Re: To Kill a Mockingbird: Chapters 13 - 18
Here Harper Lee builds the portrayal of Mayella as lying on instruction from her father. By repeating and emphasising that this was the first time Tom Robinson had visited, Mayella would be digging herself a hole if the jury were fair. But then she immediately retracts her emphatic declaration amidst wrathful snuffles of pure hatred directed toward Atticus.
And after this comes the dramatic climax of the book:
The implication is that it is impossible for a one handed man to choke and punch a person in the way Bob and Mayella Ewell have described, and that therefore they are covering up the reality that Bob Ewell assaulted his daughter.