So we reach the question: why might someone lack a belief in god, but not believe in a lack of god?
There are several possibilities.
One is that they might never have come across the concept of god before. Perhaps they're too young. Or perhaps they've been too isolated. Unlikely in this day and age, I know, but it's possible enough to demonstrate the logical error in assuming the two statements to be the same. Denial pre-supposes something to deny, and in this case, there would be nothing apparent to deny. Another possible reason is that a person might be undecided. In that case, they lack a belief in god. Again, never mind the category labels for now, whether or not you think this counts as agnosticism - just recognize that this person would lack a belief in god, without having a belief that 'god does not exist', so this is perfectly possible. A further one is that they may simply be indifferent to belief in god, for whatever reason. Or they might just think the idea of the supernatural absurd and the claims of the theist absurdly overspecific and presumptious. Again, no belief in god, and no firm belief in the non-existence of god. Just a lack of belief. In none of these cases is it necessary that the person positively believes or claims to know that god Does Not Exist.
My own reason for lacking a belief in god but not believing in a lack of god can be summed up in an argument that runs along this path...
- It seems realistic to suppose that there are limitations to human comprehension.
- We are not in a position to be able to say "this is all there is."
- God is a human-posited Being that is supposedly beyond human comprehension.
- If a realm beyond human comprehension exists, then by definition we cannot say or know anything about it.
- We can say nothing about the non-existence of a god.
This by the way is pretty much how I'd have responded to Douglas Adams' atheist comments - a man who I otherwise greatly admire, but who I suspect, on this issue, allowed his over-enthusiastic defiance of/rebellion against religion to carry him into a more strident but intellectually shaky position than he might otherwise have adopted.
If you're not already bored to tears, this will finish you off

Those 5 points in more detail:
Point 1) It seems realistic to suppose there are limitations to human comprehension.
Brain capacity/cognitive ability is one thing. We will probably make great advances in this area in centuries to come, but for now it places a constraint on what we can know. However, irrespective of brain capacity, there are other limitations placed on us from living in the Universe in which we do. Some of these limitations - such as the physical constants, the speed of light, etc - may place a fundamental limit on what we're ever able to
discover. But even if we were able to conquer those physical constraints, there are other factors that may place a fundamental limit on what we can ever
comprehend. Being creatures of a very specific number of dimensions of time and space, it seems reasonable to me to think that, beyond a certain point, humankind is as destined to ignorance as the fictional characters of Flatland. Consequently.....
Point 2) We are not in a position to be able to say "this is all there is."
Yes this life may be all there is for humanity - this organic body, this brain-based personality and this strange attribute we call consciousness may be all there is for each of us for the few decades that we're lucky to be alive. Yes the observable Universe may or may not be all humankind can ever observe. But it does NOT follow that all the things we're capable of discovering or comprehending represent ALL of "everything" there is. Because, by definition, we would never know.
Point 3) God is a human-posited Being that is supposedly beyond human comprehension.
If we are defining god as the ultimate Being, as we usually are, he would have to encompass not just the realm that we can comprehend, but also the realm that we cannot - not so much in a different parallel realm, but rather in a greater more all-encompassing realm. Our inability to comprehend infinity tells us just how far beyond human comprehension such a Being must be. Generally, when we're talking about the concept of a god, we're talking about something and/or somewhere and/or some-(?) that we accept as being fundamentally beyond our ability to comprehend.
Point 4) If a realm beyond human comprehension exists, then by definition we cannot say or know anything about it.
Yep, the famous tautology. We cannot say what this aspect of reality contains, and we cannot say what it does not contain, because we are incapable of knowing anything about something that's being posited as being beyond human comprehension, including whether or not it even exists. The entire question is off the page.
Point 5) We can say nothing about the non-existence of a god.
Since we cannot know anything about the area beyond human comprehension, and since god is supposed to occupy such an area, we cannot possibly confirm his absence (nor his presence, but that's not the point of this particular argument).
As such:
- I label myself an atheist because I do not believe in God, I have no belief in God
- But nor do I have a belief that god 'does not exist' - because I feel that question cannot be answered by any of us.
- By placing the concept of god outside the realm of human comprehension and evidence, I am asserting that this hypothetical being is unknowable. By doing this, I am also an agnostic. Anyone else who places god outside this realm (which, I should think, is almost everyone in the modern world) will, by my reckoning, be similarly agnostic.
But as I say, category labels weren't the main point of this post. If you think I'm not a true atheist, then that's entirely up to you, but be aware that many people who share my position also call themselves atheists and refuse the narrow definition of atheism foisted on them by self-labelled agnostics and theists. The main point here was to elaborate on the specific nature of my position and to let you know what I am claiming and what I'm not.