Probably because they're aware that saying "I, as an athiest have a belief in XYZ" has nothing to do with atheism and is in no way universal...? You can be an athiest and believe in unicorns, you can be an athiest and believe that we're all descended from David Icke's long-lost uncle.
Depends on your definition of belief.
No. It's a tacit admission that said owner of those lack of beliefs does not believe in god(s). Things other than that are outside of the sphere of any kind of theism.
Most athiests came to their conclusion through their own personal experiences, pretty much the only thing they have in common is a shared lack of belief inexistential deities. If you want to know more about them, ask them