Baby smaller than 12 week?

I went for my "12 week scan" but the lady said I'm only 10 weeks?

I made sure we was clear on dates because technically baby is 10 weeks plus 2 weeks from last period.

But the lady that scanned me said I'm 8 weeks plus the 2 weeks.

Heart beat was fine, baby bouncing around and everything.

But I know exactly when I ovulated and conceived due to us only trying that once (after pretty much given up).

I took a test before my period was due and I had a faint line. Then a digital test that said I was two week.

I also asked if it was just a small baby and apparently all babies are the same size at this stage?

If this is all true then how did this happen? Or is she wrong?