Confused...

I am currently between 7 and 9 weeks pregnant and I had my first ultrasound last week. The reason I am unsure and confused is because when we were TTC I charted and tracked bbt and used OPKs so I know exactly when I ovulated. According to when I ovulated, I should be about 8 weeks 3 days. According to the ultrasound tech, I am 7 weeks 4 days. Could the ultrasound tech have made a mistake? Do some babies grow slower than others? Did I miss something and miscalculate how far along I think I should be? I am just very concerned with thinking I should be a week further along than what the ultrasound tech came up with. Thank you all for your help!