Baby measuring behind at first US

So I thought I was 8+6. I was TTC and using ovulation tests so thought was pretty accurate with my dates. Had a private scan today and baby is measuring 6weeks but with a strong heartbeat. Was told the yoke sack and fetal pole were seen. Sonographer told me that if I walked in off the street unsure of dates she would say congrats your 6 weeks with a healthy baby. I am so confused. I have to wait a week for another scan to see if there has been any development. She even gave me the statistics that 97% of pregnancies where you see a heartbeat on an US are viable pregnancies. I can shake the feeling that I am measuring almost 3 weeks behind and that baby has not grown in 3 weeks. Then if that was the case would heart keep beating? Has anyone ever had this situation when you are pretty sure of dates? Thanks 🙏