Dating scan help

I just had my dating scan. My last period was 13th January, which made me 7w2d at scan. The scan has me now due a week later, so I guess I’m 6w2d. Heart rate was 124. I’m pretty sure I ovulated a couple of days later, so I was expecting to be a few days out. The Dr doing the scan said everything looks perfect for being 6w2d. (I see my dr tomorrow but the worry is getting to me).

I’m worried as I got a positive on clear blue digital at 3weeks (according to the dating scan). I didn’t think it was possible to get a positive this early? Does this mean there’s a problem with my baby not growing properly?

How accurate are dating scans- are there a few days of variability?