I think it's all about context. I know of DF'd and inactive ones, that if asked by a census taker, would reply they are JWs. I know of active publishers that actually never go out in service. Do any of these people really believe in everything the bOrg teaches? Probably not. But they would still describe themselves as JWs in certain contexts.
So, even the numbers that the bOrg collects only really reflect those that would pass certain criteria. Some level of belief, some level of attendance, turning in some hours (accuracy varies). But these people still consider themselves JWs.
As to whether the organization cares about accuracy, I actually doubt that. They are so far down the honesty rabbit hole they probably just do their own thing. If they try to create the impression that the bOrg are growing (somewhere, not everywhere) then the believers will still give money. If they were to disclose (if true) that the bOrg is shrinking, the generous ones might quit giving. That's just human nature. Humans love numbers and they love numbers that get bigger. Ask any marketing manager or financial planner.
I don't think it matters. On a true, mental level, I doubt there is anywhere close to 8.5 million. I am also sure the numbers reported by the Mormons and the Catholic Church are inaccurate as well.