Amemoir describes past events, but it is not considered a historical document.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

Read this excerpt from leah missbach day's foreword to wheels of change.bicycles have long played a role in my life. as a young woman, i rode one year-round before i had a car. but it was later in adulthood that the bicycle became more than a source of transportation for me. the bicycle actually began to truly shape the way i saw the world.what is the author's purpose for including this in the foreword?