I dunno,
There's not much we can do about the inconsistency between registration date for the current active forums (2008-) and including old posts in the post count.

There was another inconsistency I found out a few days ago, but I've forgotten.

Maybe the old registration date (if possible) could be in brackets as an addition somewhere on the profile page, so people are aware that a particular user was a member of the old forums and so there is bound to be an inconsistency with the posts per day stat.
Because, I don't see much of a way of fixing it completely, and it may not be needed anyway.

Iguess there's always the trick of setting everyone's date to their 1st post ever, but then there are people who signed up months before their 1st one, so Netham45 would have to skip anyone who signed up long before their first post