People differ on what the correct term is. I can't disprove leprachauns, but I'm not agnostic regarding them. Without a belief in a god (atheism) isn't the same as saying god definitely doesn't exist.
Serotonin_Wraith hit the nail on the head. Can you prove leprechauns don't exist? No. Do you therefore BELIEVE leprechauns exist? No! (At least I hope not.) Do you hold out the possibility that they might exist? Again no.
We find it easy to dismiss leprechauns, unicorns, Greek gods and so-forth as being mythical. Why? Two reasons. One is that there is NO EVIDENCE that they exist or have ever existed. Mount Olympus is just a jagged rock, not the home of the gods. A rainbow is light being diffracted through water vapor in the atmosphere, there is no leprechaun with his pot of gold at the end of it.
The second reason, and in many ways the most important reason, is that we were TOLD by our peers, our teachers, and our parents that this stuff was hogwash. So we find it easy to say that not only do we not believe these things but we also don't rationally hold out any ideas of their being proved true in the future.
This second reason is what makes God different. In reality, there is NO MORE EVIDENCE that God exists than there is for leprechauns. However, it has been hammered into our wee little heads that God DOES exist. Our peers, our teachers, and our parents are all believers, because they were taught to believe.
That's what makes it so hard to let go of the God concept and be an atheist. It's also why we are having an argument over what the terms "atheist" and "agnostic" mean. No one on this board is "agnostic" toward the Greek gods, we are all "atheists" towards them. I feel the exact same way toward the Bible God, and for the same reason-no evidence, and this despite what I was taught.
CyrusThePersian