What is "truth value", then? Atheists in the broadest sense and agnostists don't deny that there is a God.
If you want you can research this to verify that "truth value" is the right term here (I'm not completely sure). However, My understanding is that if you say something has a truth value, you are saying that there is a possibility that it is true. Thus:
If you are an agnostic, you lack a belief in God, but you believe that God could exist. You don't lack a belief in God due to believing that he doesn't exist, but rather you don't believe in God for some other reason, such as an unwillingness to accept fideism (literally "faithism", which is the idea that belief in God can be justified by faith).
On the other hand, Atheists lack a belief in God with the motivation that they don't believe that God exists. Like agnostics, they lack a belief in God, but it is not backed up by something like lack of evidence, such as agnostics. It is only backed up by the belief that God does not exist.
Atheism meaning "a lack of belief in God" is not contradictory with believing that there is no God, and it doesn't mean that they are not denying the existence of God. If you do not deny the existence of God, you are an agnostic, not an atheist. Atheism specifically encompasses a group of people who are characterized by their denial of Gods existence.
Edited 6/5/2016 01:59:29