When I called myself an atheist, I was 99% sure God didn't exist, even though I couldn't prove it.
By the time I decided to call myself agnostic, I was only about 75% sure, but it was enough of a difference in my mind to change labels. I was, after all, 100% sure that I didn't know if there was a God or not, and 100% sick of trying to figure it out myself.
What had made me unsure was my research into Deism, in which a completely non-personfied God existed, different from Christianity in every way. It occurred to me one day, while reading a lengthy deistic tome full of constructs of hypothetical Gods, that if God were so qualified away, unable to affect us in any discernable, detectable way, that it really didn't matter if God existed or not, so there was no reason why I should care. I put down the book and took a 20-year nap. When I woke up, I was still divided, and did not care to waste any more time figuring it out. I channeled the spirit of Rodney King even though he's still alive somehow and decided to take another crack at seeing if we all couldn't find a way to get along. The end.