I think part of what you're pointing out is that atheism is not a belief system, and so people shouldn't expect atheists to all think the same way or believe the same things.
However, it's a nice little piece of irony that, since people who claim to be "atheists" can believe different things, they can also disagree on what it means to be an atheist. I've talked to quite a few people who identify themselves as atheists, for whom it does seem to be a belief system. For them, being an atheist includes a deep respect for science, a belief in empiricism, a responsibility to proselytize. It's not uncommon for there to be a rejection of morality outside of utilitarianism. There's usually a general belief that there's nothing to this world beyond physics, the math behind the physics, and the application of physics to build up the physical world around us. There's often an associated desire to find awe and reverence in science and physics, and to treat that as a sort of pseudo-spirituality, while talking about how stupid religion is.
I find whenever you start talking about atheism, you actually end up with a fair amount of disagreement from all sides about what atheism actually is. You're confidently saying one thing, and someone else will say something else with just as much confidence. It's pretty much impossible to have a meaningful conversation unless we can agree on our terms somehow.