It seems that the term “atheist” means different things to different people. Theists often frame atheists as being dogmatic non-believers. We are as sure of god’s nonexistence as theists are in his existence. This is a safe and familiar place for the theist to be because then they can frame the debate as one between two faiths. This is easier to wrap your head around and many a time I have encountered a “Aha! You’re really no different from me then! You are stuck in your ways as I am!” or, upon hearing me say that I’m pretty certain there is no God, “Well, then you’re not atheist, you’re agnostic!” I won’t get into why agnosticism is not threatening but atheism is.
Atheists, because they are typically well-versed in science and understand the idea of balance of evidence, do not identify as someone who knows there is no God. We believe there is no God based on the evidence (or lack of it depending on your perspective) and must, in order to remain rational, allow for the possibility that we are wrong. We can never know for certain but we can measure the evidence and come down on one side or the other.
Which actually means we have something in common with theists, because like it or not, they don’t really know either.