It refers to not having knowledge, while atheism refers to belief. Knowledge and belief are similar, but not equivalent. Some consider knowledge to be the most confident form of belief, something about which you are maximally certain. In any case, atheism and agnosticism are not mutually exclusive.

I consider myself an "agnostic atheist". I lack belief in a god, but I recognise that I can't know with certainty that there isn't one out there somewhere.

But the point is, you're not understanding the negation at play. Theist = person who believes in a god. Atheist = person who does not believe in a god. Not believing only requires the absence of a belief in god ("not guilty"), but it also includes believing there is no god ("innocent").