because the "truth" of the story is constantly changing.

The story is told by Briony Tallis, the thirteen-year-old protagonist who serves as the catalyst for nearly every event that happens. The worst moment is Briony's questionable accusation of rape against Robbie Turner, an action which unalterably changes life for every character. In Part I of the novel, readers see events and the other characters through her eyes and voice, and it is unclear (both because she is young and because we are not sure if she is being honest with us) whether her actions are innocent but misguided or deliberately cruel and misleading.

As the novel continues in Part II, Robbie's story is told, creating further confusion for the readers as events and characters are seen  from a new perspective.  

Part III and the final section, "London, 1999," are both told by Briony Tallis, but in the final section we learn that everything that was written before is a novel. What is unclear is whether her writing is more fact or fiction. The lines are blurred, which means everything we thought we knew about each character is again called into question. The only reality occurs in the final section, where an aged Briony watches her grandchildren enacting a story she wrote when she was thirteen.

Because McEwan twists fact and fiction, reality and lies, so often during the novel, readers can never be sure that the characters they meet are entirely real or fictional creations of Briony's fertile mind. Just when we suspect the story is completely true (because she is worried about the legal repercussions if her novel is published), McEwan twists things again and we are again left uncertain.