Isn't this statistic based on US census-based data (i.e., self-reported religious affiliation).
It is not unusual for the number of nominal members who report affiliation with a religion to outstrip numbers who are actively involved. Hence, the discrepancy between the Watchtower's annual figures of publishers (1.1 million?) and census data (1.9 million). It is the same in NW: Watchtower says just over 14,000 peak publishers for 2008 but census data says just over 17,000.