As Agnostic, I reject the possibility of "nothing" and hence state that the creation of the Universe must have been a God but not any specific God described by any religion. Unless you can come up with some other alternative method of Universe creation, then Atheist's stance on the matter must be existence of "nothing" and such must be shown to exist somewhere. Otherwise not taking any position is an argument, "I don't know" position, is not a position to take in an argument to the point of having a name for such a stance. If Atheist can come up with some other source of logical explanation for the creation of the Universe, such position would have to be supported with evidence or simply disprove the possibility of God.
Ok terms like Agnostic and Atheist can get mixed up and muddled. Personally when reading comments on the internet I like to apply the principle of charity. Which basically means you look for ways in which the person is making sense as opposed to ways in which they aren't making sense. For example a lot of disagreements come about simply due to how people may define words differently. So it's possible for both of us to be correct in our points with our understanding of the words being used. As an example it is possible that by your definition many people calling themselves atheist are closer to how you define agnostic. If I agree with your definition of Atheism then I agree with the conclusion. But in that case I'm not an atheist (by your definition).
Having said that I will attempt to define how I use the words and how I see the difference. This is of course my take, it's open for discussion.
Agnostic: nothing is known or can be known about the existence of God. God cannot be proven or disproven. The question that comes to my mind (and where our interpretations perhaps start to defer?) is which God? And how do you define God? For example are you talking about the christian god? or a different god? How does agnosticism relate to religions that don't have gods? To my mind agnostic means religion A,B,C,...(and so on) can't be proven or disproven. I also try to strengthen this slightly to the odds of religion A being right as 50% and religion A being wrong as 50%. Otherwise Agnosticism starts to look exactly the same as Atheism (to me). Again this is my interpretation, others will use this word differently and mean different things.
Under your definition of Agnostic you are talking about a God which is not specific to any religion, I hesitate to offer a description but lets just say an undefined creator, then how you define Agnosticism means the same as how I define atheism (more or less).
Atheist: Each and every religion A, B, C,.... is made up by humans and therefore as real as Sauron. It is known that each religion is a falsehood in the same way it is known that Uri Geller can't bend spoons with his mind. That is the strong Atheist stance. Some people find this too strong and believe that there may be a teapot behind mars so call themselves agnostic as a matter of principle. Personally I reduce this to chance, if you think the teapot is 50/50 then sure that's agnosticism. If you think the chance of the teapot is tending towards 0 then you are a lot closer to atheist than agnostic (again in my opinion - other opinions exist). In this sense atheism takes a very strong position?
How atheism relates to a non specific creator depends on how you define this creator. Even using the word creator is an issue as it introduces all kinds of bias. Atheists, in general, do believe we are here but understand nothing about the how. Literally nothing. How may even not make sense as a question.
So in short, in my opinion, saying you are an atheist is not saying we were 100% created from nothing. Atheism does not make positive claims. Atheism is merely that religion A, B, C,..... has evidence that it is baseless.
To give one last example. Neil deGrasse Tyson says he is closer to agnostic than anything but at the end of the day would prefer not to have a label. Dark Matter refutes this and using his, in my opinion rather harsh and misguided, views defines Neil as being an Atheist. Personally I can see Neil's point. It is hard to get the depth of expression from one label, as it depends how you define that label. Under one definition you may be correct and under another you may be wrong.
-- Updated February 16th, 2017, 6:15 am to add the following --
Oh, I don't doubt his feelings are genuine, but that doesn't translate into spirituality. Interactions can be had with non-personal things, but not fellowship. You cannot pray to a chemical formula, supplicate a mathematical equation, worship a hypothesis, confide in a postulate, commune with a process, serve an abstraction, or hold loving fellowship with a law. To appropriate the word "spirit" and use it in a way that impinges on its religious intent is disingenuous and intellectually dishonest.
You don't own the word spirituality, fellowship, supplicate, worship etc. These concepts existed before your religion and will exist after. They are part of the human condition. All humans experience spirituality. Otherwise you are dehumanising about a billion people who are religiously unaffiliated? As to your view on the remainder who don't practice your specific religion I don't know?
-- Updated February 16th, 2017, 6:45 am to add the following --
Irrational thought can be defined as performing an action without any purpose, a repetitive irrational action amounts to Insanity. So, the early humans concluded that there must be some purpose to the existence of their reality,
I may be misunderstanding you here. Are you saying because humans are rational then existence must be rational and if existence is rational it must have a purpose?
Also Atheism doesn't say existence is meaningless (I mean some atheists might but this is not the definition of Atheism). It might be meaningless of course, it could be that human purpose is emergent from a purposeless universe (so you can have both). In human terms it doesn't seem likely though but these are very human terms. But lets say that existence 100% does have a purpose. We still know nothing about that purpose. Why should that purpose have anything to do with humans for example.
By the way I'm running out of ways to say it. But when defining the atheist position is it worth listening to atheists? If not then you can define it however you like and that definition will logically have everything to do with you personally and little to do with millions of people. That is what I would call insanity.
Unknown means unknown.