I have a lack of a third arm.
Ah, but do you merely "not have evidence of a third arm" or do you "have evidence that there is not a third arm"? And what would be the difference?
what is there about the 'meaning' of atheism that invites such intense interest?
The issue from the point of view of
rational argument is that claims have burdens of proof.
In rational argument, there is a
burden of proof on anyone claiming a god exists.
If someone does not make claim that a god exists (or does not exist), then he doesn't have a burden of proof in rational argument. All is well. You don't have to be an atheist or a theist or whatever to attempt to prove or disprove claims made. And in rational argument, you don't need to take a position that a particular god does or does not exist,
if you aren't trying to prove anything. It doesn't matter if you are an atheist or theist, you can still take a non-positon in rational argument for the purposes of discussion or debate.
The problem is when people start to make unprovable
claims about who or what they are.
So if someone starts an argument for the existence of a particular god and then says, "By the way, I'm a theist." It's sort of irrelevant. You don't have to be a theist to try and make a rational argument for the existence of a particular god. The discussion derails into the question of who or what people are and if they are what they claim to be.
From the point of view of rational argument, when a person claims to "lack belief in a god or gods", it's not particularly relevant either. It's not a requirement to rational discourse. But it is taken to mean forgoing a claim that "a god or gods does not exist" and, therefore, foregoing the corresponding burden of proof (by not claiming anything). Some people take this to mean their god claims no longer have a burden of proof. But this is not the case. In rational discourse, a burden of proof remains until rationally fulfilled.
Confusion occurs in many ways. For example, in rational discourse it is important to agree on the meaning of terms and phrases used. When people cannot agree on the meaning of a word or phrase it inhibits rational discourse. In rational discourse, when people are discussing what people actually are or are not, it becomes important to know what people mean when they say they are "atheists".
Another way it can arise is when someone makes a claim such as "Everyone is born 'lacking a belief in a god or gods'." As a claim, it has a burden of proof. So it can be confusing when, on the one hand, a person claims "lack of belief" as a way to forgo burden of proof (related to a god claim), but, on the other hand, makes a claim that does have a clear burden of proof.