Who can say for sure? ... If the WTS have any detailed breakdown of the figures, they certainly are not sharing it.
IMHO.. they may be a mix of third world converts from other Christian faiths who cannot give up their aspirations to go to heaven... and some established dubs who do have that narcissism that Finkelstein was talking about.
NB 8500 to 20,000 is a hell of a jump when you see it like that.... for a group that is supposed to be reducing !