There is an inherent advantage for the believers when using the word 'know' in debate. The atheist holds that in order for something to be true, it must be demonstrable through observation and scientific experimentation. A falsifiable experiment is necessary to actually disprove something. From Wikipedia.
"Falsifiability or refutability is the trait of a statement, hypothesis, or theory whereby it can be shown false by way of some conceivable observation practically possible to achieve....
"By the problem of induction, no number of confirming observations can verify a universal generalization, such as 'All swans are white,' yet it is logically possible to falsify it, as by observing a black swan. Thus, the term falsifiability is sometimes synonym to testability....
"In falsificationism, an unfalsifiable and thus unscientific theory is not intrinsically false or inappropriate, however, as metaphysical theories might be true or contain truth, and are required to help inform science or structure scientific theories. Simply, to be scientific, a theory must predict at least some observation potentially refutable by observation."
Simply put, it is impossible to scientifically prove anything as universally true, but it is possible to provide a scientific test that in theory might prove something as false. All the scientific testing in the world cannot prove God does exist, nor is it possible to falsify his existence. The hypothesis "God exists" is not scientific, nor is "God does not exist."
For this reason, I can see where the agnostic comes from, but not the atheist. The agnostic does not believe God exists, yet reserves ultimate judgment. The atheist on the other hand feels that they can somehow disprove God's existence, though such is scientifically impossible. In other words, they are contradicting the only source of truth they even accept: scientific experimentation.
Something I have noticed, and it's just an observation and may not be correct, but I feel that more atheists tend to have a chip on their shoulders than agnostics. It seems that because 'Mom sent me to Catholic school' or 'Bible-thumpin' George W. Bush started a crusade against Islam' or 'Evangelicals won't accept homosexuality as an alternative lifestyle,' therefore 'because I disagree with what some religious individuals have done to ruin my life or poison the world, God cannot possibly exist.' One may use this as evidence in their personal quiver, but still cannot actually disprove God. They may only support their theory, but they cannot 'know' that God does not exist.
Religious persons on the other hand are liberated in this sense. Their sources of knowledge are not limited to the scientific method (though they may be limiting themselves in other ways). They believe that God can prove his existence to them, and that they can 'know' he is real. The scientist may dispute this method, but the very fact that it is accepted on faith and not on scientific proof allows for a claim to knowledge, even if the non-believer disputes the reality of that knowledge.