Why can people tell that I am an atheist without me telling them?

This is a recurring thing in which I will meet someone and get to know them, without talking about religious views at all until the moment that I say I am an atheist they will say they knew it already. Once at a health science camp I was talking with a couple of roommates about religion and my RA said that out of everybody in the room he would pin me first as the atheist. Is there a certain type of mentality or mindset that causes people to be able to tell? Does this happen in your experience?