Is that a bug? I'm not sure... but I've noticed that in my profile (and looking at others profiles) the Groups we belong to, them all say (72,431 members).
It's a bug, and other people have noticed it already.
it might help if you named the topic after your bug. XD
But then again, I personally can't find the tread talking about that bug (since there's no search) so I wouldn't have known that it has been reported before. xD
But in the future, just report everything you notice that seems awry. If it's not a bug, no one will be angry at you. They'll probably thank you for trying to be helpful. ^^