Originally, OT posts counted, but it was decided that many were only posting in OT but having a huge post count. this made some look like they were helping a lot of people, but they were only hanging out in OT.
The decision to not let OT posts count in the shown post count was made a long time ago, and it has stood ever since. I kinda like it, because it shows who is really active in the forums and who is not.