Agnosticism and atheism are not mutually exclusive concepts.
If you don't believe in any gods but recognize that their possibility cannot be 100% disproven, you're an agnostic atheist.
If you believe that gods definitely 100% don't exist, you're a gnostic atheist.
If you believe that gods definitely 100% exist, you're a gnostic theist.
If you believe in gods but recognize that their existence cannot be 100% proven, you're an agnostic theist.
If you have no concept of gods whatsoever (e.g. newborn baby), or are completely apathetic about anything pertaining to theism or atheism, you're an implicit atheist.
Wiki on atheism:
http://en.wikipedia.org/wiki/Atheism
People who identify as non-theist or non-religious or secular are atheists who think the connotations of the atheism label are too negative/strong/polarizing to associate with. "Atheists are douchebags! Me, I'm a secular humanist non-theist, nothing to see here."
So going back to an earlier question, what makes a person become atheist from the agnostic position? And why are both equally reasonable positions?
edit: Except you say they are not mutually exclusive positions, so I'm not even sure how to phrase the question.