The only reason I know of for definate why your post count would drop would be ecause of Sunday Chat or a deleted thread. As we delete threads very infrequently, it would mean you'd posted some 120 posts in one thread (e.g.)
Just a thought..... isn't there a Grapevine archive? I've not really gone that far back before, but if a thread goes into archine (so can till be red) maybe it drops off your post count?
I can see what you're saying though Pikie - you're worried your content has disappeared.
If we delete individulal, there are good reasons for it and we pm the person involved to explain why it was deleted. If we delete a whole thread we can't possibly PM everyone who took part