Due date way off from LMP due date

Deja

I just got my first ultrasound done and based off of my last missed period, I would be 12 weeks 3 days. My ultrasound says I'm measuring 10 weeks 4 days and instead of my due date being Aug 20 it would be Sept 4 😭 With my first 2 pregnancies, my LMP lined up with how baby was measuring. I'm so confused! Has this ever happened to anyone? I could understand a few days difference but not WEEKS!