Post counts are usually counted by their existance...
There are 2 possible options:
1. GHDpro made an "invisible" board sektion ( Invisible for Normal users and maybe mods ) where all the old stuff from general chat ( and other forums too maybe, dunno exactly ) are put, either trough a bbs cleanup settin ( in many forums you can say that threads that are older then ### are moved into ### ), or per hand. And that invisible board is set on "dont coun't posts" ( and that again is an option that is mostly available in forums like this one~ ).
Result: Old threads are moved there and arn't counted anymore, but they are still there so you can see it in the profile... just not counted.
Option 2: They are being deleted from the boards, but a "clone" is also noted in the Database of the forum.
That means it isn't counted anymore, but still exists in the database.
So it could be possible, and i'm still guessing that it has something to do with the forum's cleanup system
