Actually it would be more accurate to state that a high percentage of atheists are agnostic.A high percentage of atheists seem to be agnostics?
A-theism is literally "not-theism". So everyone is either a theist or a not-theist (atheist).
Theism is a category to define all people who believe in the existence of one or more deities. Atheism is a category for everybody who does not fit into the category theism.
Agnosticism deals with the question of knowledge, not belief.