WhistlingJack suggest we snip this out of Tech help and move it to WSI and as it is does crop up I thought it worth its own thread.
It seems to happen sometimes when e.g. a double post is removed and somehow something doesn't update to say it no longer has as many posts as you thought.
I believe it can be fixed by resynchronising the database (which happened a month or so ago - when the post counts dipped) but I'll look into it. If it is a problem then we can get the database resynchronised but I believe it is a lengthy procedure (given the size of the FTMB) and so TPTB probably don't want to be running it too often.
So keep an eye out for problems and report them here.