There is an obvious explanation of the survey numbers exceeding the WT figures. It is the nature of the JW religion to have many who identify with them and hold to the basic doctrine because they were brought up in the faith but no longer comply with the 'conservative' WTBTS criterion for returning a publishers report. This includes faders and disfellowshipped who still believe.
It is however to my surprise from reading here how the conservative figure itself disguises the fact that many JWs turn in false reports to maintain their identity and credibility in the congregations.