The memoir as a somewhat indistinct form is absolutely true. So many of the memoirs I've read, and the ones I have gravitated toward most, somehow upend what I expect from memoir and the project seems greater than just the exposition of a life.
One thing about humans is that we all have them - lifestories. We live by and through them. But writers of memoir are particularly good at bringing literary strategies and form to experience (at least the good ones are).