My guess is that this survey isn't very accurate due to the small sample size. Either that or they got a good number of inactive JWs who were lying due to guilt over their poor meeting attendance.
Don't be too hasty in your "guess" OneEyedJoe.
It is not at all unusual for surveys of JWs and census results to show higher numbers of people who identify as JWs than who attend meetings and/or witness. These include individuals who may well be "inactive" from the organization's view but the important point is these individuals still consider themselves to be Witnesses or intend to go back. This phenomenon occurs in other religious groups as well.
Even disfellowshipped individuals may still identify as being JWs - of course with the intention somewhere in their head of going back some day.
Census data also includes children of JWs - even babies - these inflate the number of actual "active" JWs (by the organization's view) - so we end up with way more JWs than who attend meetings and/or witness to others.