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!
The part about "assuming" that you ovulate at 14 days after your period is right. My early ultrasounds were the most accurate for both of my sons. Later I had an ultrasound that pushed my due date up by 3 weeks because my baby was big! My first was 9 1/2 pounds and 23" long. I went into labor on my due date and had him 9 minutes after midnight on the next morning. 4 days is not a big difference. Could be a bigger baby. Unless you know for certain the exact date you conceived, those due dates could be a little off. That's probably why we hear 2 weeks before or after your due date is considered full term.