but you're still an atheist if you are not a theist. There's only 2 options: you believe or you don't. Saying "i don't know" is the same as not believing (positively). Not believing = not being theist = you're an atheist.
It could be broken down differently though:
A. You believe god exists.
B. You don't believe god exists.
C. You don't know if god exists or not..
D. You believe god doesn't exist.
E. You don't believe god doesn't exist.
F. You don't know what you believe.
So you could not have a belief that god exists, but also not have a belief that he doesn't, and I don't think you should be considered an atheist for that.
I'm not sure any particular break-down is more correct than another, but I don't think you can force people to break it down the way you have chosen.
Another way to distinguish atheism from agnosticism is
1 = belief that god exists
0 = no belief either way
-1 = belief that god does not exist
Not 1 (!1) can be either 0 or -1 so simply being not 1 doesn't necessitate being a -1.