Thin Lines Around the Genre of Memoirs

I'm curious what really defines a memoir. While many writers of fiction draw on personal experience to fuel their characters and stories, some are basically re-counts of actual events. For example, in Jack Kerouac's famed books, On the Road and The Dharma Bums, he tells factual stories and accounts of himself and his friends. The only thing that is really fictionalized are the names he re-assigns to himself and his friends. However, in his book, Lonesome Traveler, which was released later and falls into the autobiographical/memoir category, he re-tells the same exact stories, only with the real names and drier prose. Beyond the name changes and the built thoughts/assumptions around each, there is little difference. Yet one is considered a memoir, while the other two are fiction.

To give another example, take Irish literature. Frank McCourt, Irish-American author of the memoirs Angela's Ashes and Tis', writes of the awful childhood that he experienced in the poor and unstable Ireland. While it is fictionalized stories, James Joyce, in his collection of stories called, The Dubliners, presents very similar accounts to the struggles and hardships of an Irish upbringing at the time. Though the differences here are more clear, I think the lines that separate memoirs from certain fiction and other genres are very fine. I know there are plenty of other examples, such fictionalized stories of slavery and the holocaust versus memoir accounts of each. Thoughts?