Not believing in god(s) and believing that there is/are no god(s) are two separate categories. The distinction is obviously missed by many and may appear to many others as a subtle one, but it is actually a very significant distinction.
Someone raised in a remote location where there does not exist any concepts of god(s) therefore does not believe in god(s) (because the concept does not even exist for him or her) but clearly cannot believe that there is/are no god(s) (again, because the concept does not even exist for him or her). One cannot believe in the nonexistence of the concept of something if one is not even aware of the concept in the first place. That is the clearest example of the distinction.