This would be true if we were talking mathmatical. However, the human language is not mathematical. Words are generally related to their constituents, but you cannot typically deduce a word's full meaning by evaluating only the meaning of its roots.

The word 'atheist' is a good example of this, because evaluating the prefix and the word theist would lead you to believe that atheist means "not theist", but this isn't the case. Atheist is a lack of belief in gods. Theism is a belief in one or more gods. There is still a generally accepted hazy area in between where an individual can be not atheist or theist, by denying the possibility of having any knowledge on either account. This is more or less like fuzzy logic, which your mathematical analogy won't account for.

Additionally, the definition of atheist isn't well accepted either. Lots of people believe it means a belief in no gods. This is widely accepted enough that you will find this definition in lots of dictionaries. In truth, I think it is also closer to how the word is intended, as a true lack of belief in deities rather than a belief in no deities can only occur if you have never been introduced to the concept of a deity at all. You cannot disprove the existence of a hypothetical god, so on some level you must simply believe they do not exist. This definition then clearly leaves room for additional positions that are both not atheist and not theist, even without fuzzy logic.