Exactly the same thing happen to me
I had this happen. When I went in for my first ultrasound, we thought I was 10 weeks based on my LMP, but based on the measurements and development of the baby, the doc said I was exactly two weeks behind, so 8 weeks. Now I'm 36w+5 and they've stuck with the same due date the entire pregnancy. Everything is going well. You most likely just ovulated later than expected. That's what happened with me.
We had a similar thing happen.... I went in for a dating ultrasound at what we thought was 8 weeks... they dated me back to 6 weeks. Then at my 12 week ultrasound I measured at 14, and my 20 week I measured at 22... So we went back to my original due date. 6-8 week ultrasounds are hard to tell because they are so tiny, and all baby's develop at a different rate!
I ovulated late and based on my LMP should've been 9+5 at my first appointment but was 7+3. I'm now 30 weeks and everything is perfect!
It is actually pretty common. Some women just ovulate later than others. If the ultrasound looked good I wouldn't worry at all :)