CA - I think the crux of the matter is not the degree of certainty. Atheism is more of a passive or negative position, "disbelief" rather than a positive "there is no god" position.
Most people are either believers, or as Dawkins said they "believe in belief".
Atheists just go a bit further and say something like, "I don't believe in god due to lack of evidence and I don't think belief or faith is a virtue I am missing out on".