Because the term "lack" implies that someone either holds a belief that God exists or they do not hold a belief that God exists. This goes for any belief. Being undecided means that you don't hold a belief in God and you don't hold the belief that God does not exist, but, nevertheless, you still lack a belief in God.
If you do not lack a belief in God, then you hold a belief in God, and, thus, you are a "theist". If you do not hold a belief in God, then you "lack a belief in God", and you are an atheist. Those undecided still do not hold a belief in God, and, thus, by definition, they "lack a belief in God", which is all that is necessary.