They usually base your due date on your LMP and that is assuming that you ovulate 14 days after your period starts. You must have ovulated early, therefore your baby was conceived a little bit early, so they moved your due date up a few days. That's how I understand it to be, but feel free to correct me if I'm mistaken.
EDD with baby #1 for hubby and I September 21st, 2003! It's A BOY!