Was the sec on December 27, 28 and 31 all with the same guy? The calculator I use gives December 29 as the most likely conception date for that due date.
How did you get the due date, was it from an ultrasound? And if it was from an ultrasound, was it from an early ultrasound, say, before your 8th or 9th week?
It's not really possible to determine an exact due date just from when the first day of your last period was, because although if a woman is very regular in her cycles, like clockwork every 28 days or similar, it can be a useful predictor, many women are not that regular. And even regular women can have a cycle where they ovulate at an unusual time. Ultrasounds are much more reliable, though they also (to be useful) have some limits. You can't use an ultrasound that was done later than your first trimester to pinpoint things, since babies grow at different rates.
Therefore it would really help if you would say how you know you are due September 21, how you got that information and how early in your pregnancy you got it.