Many churches count among their numbers people who stop going to church, no matter how long they haven't attended their services. This seems to be especially true of Mormons and Catholics. Whether this is optimistic or opportunistic, many people are disturbed by this practice as it inaccurately depicts congregation numbers.
Look at it this way, if a person is baptised into the Mormon church at the age of eight (or the Catholic church at the age of 8 weeks), but never goes to church services, marks the "atheist" box on census forms, yet never bothers to officially remove their name from church records, will they die a member of that faith? If you ask religious authorities, yes. This may be technically correct by the Church's standard, but by any practical measure they never really participated in the religion, and so should not be considered as members. The same should be true of one who leaves a given church later on in life. If they say they no longer wish to be a member, then that should be the end of it.
Some churches (again, look at Mormonism) intentionally make it quite difficult to remove one's name from their records. In Mormonism the process of removing one's name from church records consists of disciplinary hearings with various church leaders, during which one is asked to denounce the church and its teachings. It is as awkward as it is unnecessary, so many who leave the church either leave without removing their names or threaten legal action if the church insists on going through the "proper channels" in order to drag things out.
In related news, Atheist Alliance International is conducting a "census" where former believers can participate and offer information about their backgrounds. Note: the servers are having issues, so if you can't get to the survey, try again later.