I think it is more that an atheist believes that there is no god whilst an agnostic simply states that they do not know. Obviously a theist believes in a God but, in common with an atheist rejects all other alternative gods.
Whilst an agnostic may argue that theirs is actually the only logical position an atheist would probably counter with the position that they find the likelihood of there actually being a god to be vanishingly small.
Whilst an agnostic may argue that theirs is actually the only logical position an atheist would probably counter with the position that they find the likelihood of there actually being a god to be vanishingly small.