Here:
Do you believe in God? Y/N/Maybe
Agnostics choose maybe. That is neither yes or no. Replace Y with Theism, N with Atheism, and Maybe with agnosticism, and there you have it.
You seem to be confused because to you, the question has only two answers. But it has 3. The third being 'maybe', 'i dunno', or 'who cares'. Many people find this answer unacceptable. But to me it's valid and indicates that the person in question either doesn't care or doesn't want to be grouped with one side or the other for whatever reasons. And that's fine. There IS a middle ground.
EDIT: finally, being undecided =/= without belief. it is neither with belief or without. why is this so? because their definitions are completely different.
Do you believe in gravity? Y/N... Maybe?
What about, do you believe Obama is the current POTUS? Y/N... Maybe?
No matter what you answer, gravity still exists and works on you, and Pres. Obama is still... Well, yeah, PRES. Obama.
The point is, asking your belief on something is asking your opinion, not fact. Therefore, you can't have a middle ground.