Measuring 10 days off

I had my first ultrasound yesterday. I thought I was 7w3d based on the date of my LMP.  We could see the little blob on the screen and even saw and heard the heartbeat!  But the baby is measuring at only 6 weeks. I can understand being a few days +/- but 10 days?  Makes me concerned to be so far off.  Dr said I probably just ovulated later than normal or my dates are off. I know for sure the date for my LMP because we were really trying so I have perfect records.  I go back in 2 weeks to check again.