You're not really atheist, you just want to be contrary. You really believe, and someday you'll admit it.
There are many popular myths about atheists which portray them as not really being serious about disbelieving, while other myths suggest atheists are in denial about what they really think. This myth combines both and presents them at the same time. First the atheist is only proclaiming atheism in order to be "contrary" with others while at the same time denying what they really believe someday, they will admit the truth and will give up acting like they don't believe in God.
Accusing someone of being "contrary" is to accuse them of being stubborn, headstrong, obstinate, insubordinate, and rebellious. All of these synonyms communicate something important about the speaker's attitude towards the atheist and, perhaps, atheists in general. They are "stubborn," which means that they resist listening to the religious theists (almost always Christians) who are trying to be considerate and reasonable. They are "headstrong" in that they refuse to accept the reasonable arguments of people who know better.
They are obstinate in that they refuse to admit what they really think and all the atheist arguments they are offering are nonsense. They are insubordinate in that they are refusing to obey the proper social, moral, and religious authority figures in their lives. They are rebellious in that they are pretending to be atheists in order to set up some sort of opposing belief system to the religion they grew up with.
The accusation that atheists are in denial is one that I've dealt with in more depth elsewhere, so it should suffice to say here that few people are ever in a position to make a claim like this with any degree of authority or reasonableness. It's true that some people can be in denial about what they know, but you cannot accuse someone of this without very good reason, and it's almost never the case that a person has good enough reason to say this about someone merely because they say they are an atheist.
One difference between being "contrary" and the various synonyms above is that "contrary" carries a connotation that one is even less serious than if one is simply rebellious or stubborn. A person who is "just being contrary" is arguing for the sake of arguing and contradicting others just for the sake of getting a rise out of them. Thus the atheist is just pretending to reject the existence of God for no better reason than to annoy others and become the focus of attention.
The entire purpose of this myth about atheists, like so many other myths, is to implicitly deny that the atheist is worth listening to or has anything worthwhile to say. A person who is just being "contrary" is not offering serious arguments that convey real critiques of the subject in question. A person who is in denial doesn't really believe what they are saying, so of course they don't have to be listened to. This myth manages to double up on the reasons for not treating atheists seriously.
This effectively allows people to completely ignore atheistic critiques of religion, religious beliefs, and theism. They don't have to answer any of the objections and questions. They don't have to defend their own beliefs or support their own claims. They never have to stop and think seriously about whether the beliefs they were taught in childhood are really true after all.
Instead, they can continue on as if nothing has happened no one has come along to disturb their neat and orderly religious world. That's unfortunate, because in dismissing another person without thinking about what they have to say, they lose the opportunity to learn something new and maybe grow a little bit themselves.