i asked chris the same question and this is wat he told me...
Hey:
What happened is that once you changed your preferences, it takes affect towards the *new* threads you respond to after that. So from here on out, any new threads you post at won't send emails to your account alerting you of a reply, but the threads you responded before you altered your setting will continue to. How you stop that, though, is by unchecking the box that says "Notify me when a reply is posted" under POST OPTIONS every time you go to reply to previous thread that you already have.