We expect both to give us personal information and let readers see into our thoughts and actions.
The purpose of the two books differs. The memoir focuses on specific instances. The White House Years would be a good example. Memoirs usually cover a brief span of time, and their main purpose is to draw the reader's attention to a specific theme or circumstance.
Look at this title: We Were Soldiers Once . . . and Young: Ia Drang—The Battle That Changed the War in Vietnam. Harold G. Moore wrote about a particular period in history and his subtitle explains his purpose.
The book I wrote for Don Piper called 90 Minutes in Heaven has some biographical information, but the focus is on his death-and-return-to-life and his recuperation. That's properly a memoir.
However, I wrote the autobiography of Dr. Ben Carson called Gifted Hands: The Ben Carson Story. The subtitle makes it clear that we tried to cover his entire life from his birth until we wrote the book. Gifted Hands came out in 1990 and has remained a good, steady seller. The 20-plus years since publication doesn't make it less an autobiography.
I'm careful to make the distinction
that a memoir relates to a limited
period of time and events.
The whole life is an autobiography.