8 week ultrasound=5 week 5 day baby?

Cheerbuddyloves her hubby

I am hoping someone out there can explain.   At my 8 week 4 day appt, my baby showed up as 5 week 5 days.  My dr looked at my tracking and decided I must have conceived Oct 7.  I had a bfp Oct 8 so that can't be right  The heartbeat looked good and I feel great.  No spotting.  What's up with all that?  Blood work and levels looked great.