Yeah, I checked that out as well. It will only account for posts that you can see when you're evaluating someone else's post count.
I haven't bothered to check if there is a delta between the posts that are listed in the box and the ones listed in the stats page.
The board percentages totally give away when a user is posting in a hidden forum though.
Back when we first set up the hidden boards this question came up, and people seemed to prefer keeping the rostered boards counting toward their total posts. Good reasons, pro and con.... but everyone liking the higher post count seems to be the kicker

And, of course, there's the lounge...