There's a fundamental disconnect between atheists and the word "atheist". Part of this is because there isn't complete agreement on what the word means.
Lack of belief in God's existence is more specific but still leaves the door open for interpretation because the phrase "lack of belief" is ambiguous. It might be a matter of degree or preponderance of evidence and not an absolute statement of disbelief. A more definitive position would leave no doubts:
It's been my experience that most long-time atheists do not deny God's existence and adopt a more scientific stance which is willing to consider any argument or evidence that might change their minds. They don't want to make claims they can't back up. They understand the need for rational integrity. Just as the unqualified claim of God's existence is an article of faith, so is the unqualified claim of God's nonexistence. There's no evidence either way. An atheist can claim that there is no evidence for God or the supernatural (and never has been) and that there's no compelling reason to believe there ever will be. But there is a possibility -- however vanishingly small -- that there could be. A creator God is not an impossible proposition given what we know thus far. Existence . . . whether it's God's, the universe's or ours -- is an ineffable mystery; the greatest mystery of all. Certainty is an illusion and a claim that science is careful to avoid. Our understanding of the universe has undergone multiple paradigm shifts and will experience more in the future.
Whether you're absolutely certain that God exists or does not exist, you're pretending to know facts you have no access to.