Amemoir describes past events, but it is not considered a historical document. why?

Because a memoir has a literary value, the person writing a memoir doesn't have the intention to present every historical even in the exact same way it happened. A memoir actually represents the writer's memories, his own experience, it is not written by a historian with a view to being published as a history book, but rather as a work of fiction.

You need to provide more information because i do not know what story it is or character this is referring to.