Agnosticism is often thought of as scientific because it appears open to new ideas. It implies that there is a possibility that evidence might exist that would prove that deities are real; we just haven’t found that information yet and we may never find it. Well, atheists aren’t that different. They haven’t seen evidence for deities and therefore they do not believe in them. But every true atheist is philosophically scientific, and new evidence will lead to new conclusions. So, if ample (and credible) evidence was uncovered that proved that deities do exist, then all true atheists would become theists. That’s no different than people claiming agnosticism.
There is, of course, that old argument that the difference is over belief and knowledge. Well, if you believe there are no gods, that makes you an atheist. If you believe the answer can’t be discerned, that makes you an agnostic. Though one claim refers to belief, the other claim refers to a belief about knowledge. And if you truly believe that there is even a possibility that deities might exist, then you are not an atheist. One cannot claim “I know X” and “I can’t know X” at the same time without being logically inconsistent.
Having personally gone through theism and agnosticism, I fully understand why people would want to call themselves agnostic atheists. It has a (slightly) better connotation to it than atheism, it sounds more open-minded, and it appears to be more scientific. But the truth is that claiming agnosticism gives credence to the idea of theism, and that is the polar opposite of what a true atheist believes.