Ultrasound off by a week - impossible

My ultrasound at 6.4 weeks measures off by a week at 5.4 weeks. Seen heartbeat but didn't get a BPM. Ultrasound tech wasn't worried but it is physically impossible given when we had sex, when I got my BFP and my ovulation tests for me to be more than 2 days behind, no chance of a full week. Next appointment isn't until 14 weeks.

Any positive stories?