The rationale of many of these discussions is often based on an unthinking acceptance of assumptions, which when examined can disclose why someone's actions often tell a different story to their professed worldview. This paradox is not atypical of contemporary postmodern society where people are seemingly comfortable to hold contradicting views.
Just as Christians are far from homogeneous in their belief ... often holding to the dogma with no reality to their faith ... so too do atheists also come in many flavors. To engage in discussion between members of different persuasion, it is helpful to be conscious of the diversity. This article has some interesting insights into the atheistic "groups" of people: HERE.