It still gives a general idea...
Yes and no. It seems that people who were part of the forum pre-crash are locked in at that number as a baseline. If say Nick and I both decided to leave DTF for two years then come back, he'd still be over 10000 posts and I'd be in the hundreds, despite the fact that on average we probably have the same number of posts/day over the last year.
As for Dublagent66's idea, I don't think the software supports that because I doubt the mod staff is looking to scale back everyone's post counts. I'm guessing the software counts your current posts plus a predetermined number (what you had at the forum crash of 2009).
Feel free to correct me if I'm wrong, but I haven't seen anything to contradict that model.