'Agnostic' literally means "without knowledge".

I have plenty of knowledge about deities. I know that some potential deities are logically self-contradictory, and thus cannot exist. I know that other potential deities are logically possible, but there is currently no evidence in favor of them. Thus, I am not an agnostic. But I am an atheist.

Just as all squares are rectangles, but not all rectangles are squares, all agnostics are atheists - but not vice versa. The categories are not exclusive. One is a subset of the other. (Assuming, of course, that you're dealing with a person whose creed is based on knowledge. I suppose someone could have no knowledge of deities yet believe in them - such a person would have awfully shoddy standards for belief, though.)

Infants are both atheists and agnostics. With time, it becomes possible for them to gain knowledge about deities - but few of them do so - and to adopt a belief in deities - which most do.