Why don't some people understand lack of belief?

[Mister Agenda]

But when talking about "God" (like the OP did), most people will assume you're referring to THE God. The one that the vast majority of people in the 21st century believe in. And you've already said you have disbelief in that God lol. So that's what people are going to think of, and that's what people mean when they get the impression that atheists have disbelief in God, rather than just not believing or disbelieving.

As of 2005, 54% of the world's population followed one of the Abrahamic faiths (about 3.6 billion people). Your mileage on what's 'vast' seems to differ from mine.

Disbelief: inability or refusal to accept that something is true or real.

I'm afraid 'disbelief' also covers both 'not believing' and 'believing not'. Disbelief is a fine description of the agnostic atheist position.

I appreciate that you're trying to clarify though, I think you've got the gist of it: most people probably do assume most atheists they hear from are gnostic atheists; but agnostic atheists can be just as vocal, though perhaps with slightly more of a tendency to be polite about it.