Great question - let me explain.
When you hide a post by a user, they still retain the (usually negative) points they received from votes on that post. The rationale is that if it's a bad post (as per votes), the user should still be penalized for it, even if it's hidden.
By contrast, the user loses any points they received for making the post itself. The rationale here is that, if you've hidden the post, you don't think it's worth much, and therefore the user should not be rewarded for it.
Still, if you know a little SQL, you are welcome to tinker with all the formulae in qa_db_points_calculations(...) in qa-db-points.php - the tilde (~) symbol is substituted with an "=..." clause for the user id whose points we're recalculating.