Creative Liberty or "A Million Little Lies"

In class this week we were just assigned a new project.  As excited as we all were (just a little touch of sarcasm- haven't yet mastered the art of an innuendo in the written word), this new project was actually intriguing.  We were told to find specific evidence of differences in Frederick Douglass's three memoirs:  Narrative of the Life of Frederick Douglass (1845), My Bondage and My Freedom (1855) and finally The Life and Times of Frederick Douglass (1892).  When reading the three separate excerpts, Dani (my partner) and I were shocked and confused to find that as time went on the narrative edits got more and more detailed.  By the time forty-seven years had past, Douglass's memoir had nearly doubled in size.  And as I have gathered from personal experience, I am pretty sure that people's memories work opposite from that and didn't understand how that could happen.

While talking about the different levels of detail in the memoirs with Dani, I remembered from about seven years ago the book, A Million Little Pieces.  It was a book about James Frey's personal journey dealing with the hardships of overcoming a severe alcohol and drug addiction.  I also remember that it was 'all the rage' and even appeared on Oprah's Book Club.  I also remember the controversy surrounding the book that surfaced later.  After checking to see if his story checked out factually, many (including Oprah) were stunned to find out that many of the experiences vividly described in the memoir were completely fabricated.  James Frey now includes an apology to readers in the first few pages of the book (see Amazon link above).  When I was a young girl I could not understand what would drive someone to do such a thing.  But now that I am older (and somewhat wiser) I believe I understand.  He did it to add a shock element. He did it so his journey would be extreme.  He fabricated all of those hardships in order to seem like a more reliable source.  By exaggerating how bad he had it he made sure that his story would not be one to be looked over.  He wanted it to be a lesson and a warning, he wanted it to make a distinct point and so he fabricated evidence in order to make it unforgettable.
 
Now, I am not saying that Frederick Douglass has fabricated his story in order to make it more believable; I truly believe that the events described in the narrative are actually those of Frederick Douglass.  But what I am unsure about is how much 'creative liberty' Douglass took when fleshing out his memoir.  In the span of forty-seven years much is forgotten yet his narrative nearly doubles in size.  How much is actual memory and how much is exaggeration in order to set the scene or make his point.  How much more willing are authors to add details that they cannot verify themselves in order to prove something?  How does the author's agenda shape their writing?