Gaps between self-described and formal membership are common, said David A. Roozen, a sociologist who tracks religion trends as director of the Hartford Institute for Religion Research, in Hartford, Connecticut. For example, 25 percent more people identified themselves as Episcopalians and 33 percent more people claimed to be Methodists than either national body counts. The gap works the other way for some traditions, however: The Church of Jesus Christ of Latter-day Saints counts almost 33 percent more people as members than reported themselves to be Mormons in the Pew study.
Most churches don't count someone as a member unless they actually attend. If someone stops attending or donating after a period of time, the church drops them from the rolls. When asked in a survey, some of these people will still consider themselves as belonging to a particular religious movement.
The LDS Church on the other hand counts as members not only those who no longer attend, but also those who no longer consider themselves Mormon. This has long been suspected and infered from the Church's own statistics, etc, but here we have yet another well-conducted study showing that the church grossly inflates its numbers.