How does that make sense? Doesn't that mean there are quite a few who both consider themselves "athiest" or "agnostic" yet still do believe in (at least one) god? I'm confused.
Example: A deist is probably going to read religious text even if they don't completely believe in them. Because a Deist believes there is a God meaning that God has some preference as far as our behavior and so they'd be looking for evidence of that preference even if they don't believe in things like miracles (think Thomas Jefferson creating a Bible that omitted any supernatural elements).
An agnostic who might lean towards a God isn't going to preoccupy his or her self with moral judgments of a God because they're not really convinced one exists. They might try to be a good person for secular reasons and "just in case" but they aren't going to go out of their way to divine the judgement of something that may very well be imaginary.
In other words: One's a seeker while the other just holds an academic belief that there might be something to seek.
(A)gnosticism has more to do with certainty than belief - someone who is gnostic is sure, someone who is agnostic is not sure. There are plenty of agnostic theists, as many religious people don't claim to be sure about the existence of god, but choose to believe or think it probable.