At the end of the book, we discover that Briony had actually been the narrator of the story.
It doesn't clearly states that she made up the last encounter with cecilia and robbie but she says: "it is only in this last version [the one she wrote in march 1999] that my lovers end well, standing side by side on a south London pavement as I walked away. All the preceding drafts were pityless. But now I can no longer think what purpose would be served if, say, i tried to persuade my readers that Bobbie turner died of scepticaemia at Bray Dunes on 1June 1940, or that Cecilia was killed in September of the same year by the bomb that destroyed Balham Undergroud station."
This means Briony has very possibly changed the outcome of the story in order to give the reader a "sense of hope or satisfaction", just like the endind of The Trials of Arrabella had clumsily done before. ("it occured to me that I have not travelled so very far at all, since I wrote my little play.")
"Atonement" is all about the differences between fiction and reality and the powers of the novelist over her story. Telling the absolute truth was "always an impossible task, and that was precisely the point. The attempt was all."
Reality is what the novel says: "But what really happened? The answer is simple: the lovers survive and flourish."
I know it is confusing and I found the ending a bit frustrating myself. But i liked this reflexion on novel writing, morals and reality, duties and powers of the story teller.
E.