Atheism has less “variety” than theism because atheists draw from a single source of information: Science and mother nature. This common source of information necessarily leads to relatively homogeneous conclusions.
Theism on the other hand has much more variety because it draws from numerous different sources of information, each founded on the unique attributes of their particular god. These differences are why everyone is aware of the numerous religions and religious sects in the world.
But because of the lack of significant differences between atheistic world views, religious believers (theists) tend to assume that all atheists think the same.