n my case, I watch the u/s tech make many many measurements (I'm kind of a nerd and like to know exactly what she's measuring, what all the abbreviations mean, etc) and each time the computer calculated estimated gestational age and on EACH body part it showed up as between 31.5-33 wks GA, rather than 30 wks 1 day which is what it should have been. But stories like yours make me doubt the accuracy of those measurements, plus I know that individual variation counts for a lot. It was just that baby was so consistently big, yk?? Hmmm...
First - Did you have an early ultrasound for dating? If you did, whatever date that scan estimated is likely to be the most accurate one. After 10 weeks fetal age, the baby's size starts to be determined more by genetics and is less universal. The more a pregnancy advances, the less accurate the dating becomes. Remember that the measurements they take are plugged into an algorithm that compares your baby's measurements against the average of lots of other babies. There is plenty of room for variation, especially at this stage of pregnancy.
In fact, with all of my ultrasounds I have measured, and the baby has measured, spot on for my known dates (and I was charting, so I know the exact day of conception) - still the technician was amazed that every measurement lined up identically with my EDD. Because they normally see a lot more variation than that, even when the dates are 100%. The point is, even the u/s tech acknowledged that measuring spot on for dates is an unusual event - not the norm.
Second - Before early ultrasound dating and for 100s of years (and even still now), midwives relied quite a bit on the quickening milestone to estimate due date. They would ask mamas to make note of the week they first felt strong unmistakable baby movement (for me, it was 18 weeks with this pregnancy) and then add 22 weeks for the EDD. How does that milestone match up for you with your original due date? How about with the one the ultrasound is predicting?