I think we are really playing with words here. You are absolutely sure God does not exist, but I cannot make the required leap of faith that being a true atheist implies and, thus, I can only call myself agnostic. Still, I am pretty sure He (or She, or It) doesn't exist.
As you well pointed out, I am not absolutely sure of anything that cannot be proved with a sound mathematical demonstration. That's about all the faith I have.
And I place it _very_ carefully ;-)