"It was not reasonable or just to hate Briony, but it helped" (215).

I had completely forgotten about how Robbie explained away Briony's actions against him. I recalled Briony's insignificant mentioning of the incident, her jumping into the river to see if Robbie would prove his love for her by saving her, but I had just sort of brushed it off. Briony didn't attribute any significance to the incident, however, perhaps simply by mentioning it she is revealing her real, suppressed reason for accusing Robbie: jealousy of his love for her sister. I had never before thought of the incident in this context before, but it does make sense as a possible explanation for Briony's actions. Taking into account that Briony is the author of this, perhaps the whole reason she concocts the reason for blaming him because it "made sense," is to disguise the true, petty reasons for her actions. This could explain her extreme guilt at the end of the novel, as she can never really forgive herself for allowing something as silly as jealousy to ruin the lives of her sister and Robbie. Her covering it up could also be an example of her intense need for secrets explained at the beginning of the novel. In his article "The Impresion of  Deeper Darkness: Ian McEwan's Atonement, Peter Matthews explains Briony's need for secrets:

At the centre of the book's narrative is a secret, an obscured truth, which McEwan uses to lure the reader into the story. Like Briony, the reader is pushed toward a moral judgment by this act of concealment, even though the information necessary to make an ethically informed decision is withheld. Each secret contains two possible destinies, writes Maurice Blanchot, "The stratagem of the secret is either to show itself, to make itself so visible that it isn't seen (to disappear, that is, as a secret), or to hint that the secret is only secret where there is no secret, or no appearance of any secret" (137). The crucial quality of a secret, in other words, lies in its form rather than its content, making the source of its attraction entirely negative. The paradoxical result is that the positive content at the heart of the secret, the evidence that can be gathered and analyzed, is effectively sidelined by the act of obscuration that frames it. (https://muse.jhu.edu/journals/esc_english_studies_in_canada/v032/32.1mathews.html)

While in his article Matthews argues a much different point (that there really is no secret in the novel and the story is merely Briony's attempt to expose her secret when, in reality, she has none - something that I hope to explore later on), the same argument can be used. Perhaps Briony admits to one secret, the possibility of her crime based on her innocence rather than the real reason - her jealousy.

Matthews, Peter. "The Impression of a Deeper Darkness: Ian McEwan's Atonement."Project Muse. 2006. Web. 1 Dec. 2011. <https://muse.jhu.edu/journals/esc_english_studies_in_canada/v032/32.1mathews.html>.