I'm with slimboyfat & Heathen
People leave the JWs and no longer identify with them, unlike many other groups. I get the impression from lapsed Catholic/Jews/Mormons and Muslims that they still identify with their faith of birth while no longer practising it. Few lapsed JWs will label themselves as such, even if they are believers, because it's instilled into them that unless you are Witnessing you're not a JW.
This would explain the difference in some of the figures.