Fictional, of course ;) Let's say a story opens with a narrator saying someone committed a murder. For example, "John Doe committed the murder, and this is why it happened." It is a 'whydunnit'. The rest of the book is how and why the murder happened. Would you be disappointed if, at the end, it turns out someone else committed the murder? Is there any scenario in which you would or wouldn't be disappointed if someone else ends up committing the murder?
Note: Edited for clarity.