Agnosticism and Atheism are addressing two different types of things. Consider the quote,
I know I saw a UFO, but I just can't believe it!

Atheism addresses what one believes (accepts as true).
Agnosticism addresses what one has knowledge of (data).

One can have knowledge of the Harry Potter plot and characters without necessarily accepting it as true. You'll notice that there's a difference between someone saying "I don't know", and someone else saying "I don't believe." Often, people have a misconception that "to know" is a more confident version of "to believe", yet this is not the case. They're measuring two different things.

Most religious people are agnostic theists and most non-believers people are agnostic atheists.