Dating off? HELP!?

Last Friday I had an ultrasound done and she put me at 7 weeks 5 days which put me a few days ahead of what I was a week before. Well I had another ultrasound today because I’m “high risk” from having multiple losses and a whole week later I measured 8 weeks 0 days. How is this? I asked her if this happens and she said it’s normal with different techs and machines sometimes dating is off. As long as it’s 5-7 days off its okay . But that seems odd to me. I’m just worried after having multiple miscarriages. Heartbeat was 171 so that is good news. Please anyone have similar experiences?