Ultrasounds Question

Amy • Tue, Mar 29

Had my first ultrasound today and it went great! According to my last period, I am 7 weeks and 6 days today but when taking measurements of the little bean today, I would be 7 weeks 2 days. The dr. stuck with the due date going along with my last period, I didn't think about until now but I'm wondering why?? Wouldn't the measurements be more accurate?