Ultrasound dating verses LMP dating


According to my ultrasound I measured 7w3d but according to my LMP I was 8w2d. What would the 7w3d be based on. I tried the measuring ovulation (I didn’t track it this month but I usually ovulate around cycle day 18) and then adding 2 weeks and I end up being further ahead than the 8w2 day as well. I don’t understand how that works. Is it ok that I “measure” 6 days behind?