Self proclaimed atheist can claim that the don't know for sure that there is no god, but they all believe that there is no god.
Se there is a fundamental difference between belief and knowing. For example I believe I am in the real world and everyone one around me is an actual person. But I do not know this for sure, there is the possibility that I am actually in a simulation, and every thing or person is nothing but virtual reality.
Google definition of atheist-a person who disbelieves or lacks belief in the existence of God or gods. As opposed to agnostic- a person who believes that nothing is known or can be known of the existence or nature of God or of anything beyond material phenomena; a person who claims neither faith nor disbelief in God.
If a knowledgeable person declares him or her self to be atheist, that person is declaring their disbelief in God(s).
Easy way to think about it, asked if God(s) exist, atheists say no, theists (for lack of a better term) say yes, and agnostics say maybe.
Atheism is the lack of a belief in a god or gods, not the belief that there is no god, even insinuating otherwise shows you don't know what it means, atheism doesn't have any set beliefs, therefore no, atheists (self proclaimed or not) do not believe there is no god, they have no belief system whatsoever.
Some do and some don't. Just like some religious people believe in the Christian god and some in the Greek gods. Just like some U.S. citizens are black and some are white. Just like some men like football and others dancing. Just like some women like cooking and some hunting. Just like some people are attracted to members of the same sex and some of the opposite.
For a more theoretical answer by me, refer to my arguments on a previous opinion poll (http://bit.Ly/1b8lJ2x).