Atheism is a-theism. Theism is a belief in a god or gods. Atheism is therefore no belief in a god or gods, not a belief in no gods. This means that many people who call themselves agnostics are actually atheists as well. The definitions are not mutually exclusive.
I get your point, and in fact I'm happy to call myself an agnostic atheist. I am an agnostic because I don't know whether there are any gods, and I suspect that I can never know. But I insist to you that I'm also an atheist, because I conclude from this lack of input that there really aren't any. It's a conclusion rather than a belief, and I'd change it if some evidence for gods turned up, but it's my considered opinion that if there were a god, there would be more evidence.
It's like being confident that you won't be struck by lightning tomorrow. You can't say it'll never happen, but it's so unlikely that you live as if it won't happen. In all probability, it never will. If three of your neighbours were struck by lightning in the same week, however, you might reconsider that position.
- SmartLX