Measuring 6cm at 8 weeks?!

BBLaws

okay, maybe I'm a little confused! So last week when I went to my confirmation appt, during my physical exam my NP said I was 9 weeks 1 day (based on my last period) BUT we went in for a ultrasound and the embryo was 4cm so they put me at 7 weeks. Today I went in for some spotting, had another ultrasound, fetus measuring now at 6cm and I am at 8 weeks. But when I do the calculations, the cm measurement doesn't add up. Are they measuring something different or a different way? The 8 weeks today, is spot on for when i conceived. But this is the second ultrasound tech that measured like this, so i think I am missing something... can anyone help me understand?