Happened here too! Was supposed to be 9 and they told us 7. I figured though because I had pcos and my cycles weren't regular. The way they first calculate the due date is if you have perfect 28 days cycles and you always ovulate on the 14th day.
I'm supposed to be 8 weeks and 4 days and when they did the sonogram it appear it was 5 weeks!
You ovulated later than is "regular" my first pregnancy was way way off. I went a cycle and a half before I ovulated. So from my period I was like 10wks and by first ultrasound I was 6wks
I'm due sept 11th and the first day of my last cycle was Dec 5th