Confused!!

So, I though I was 5 weeks 5 days today, and was in for an early pregnancy scan because of an earlier miscarriage (the cycle prior to this one!)

Anyway, the ultrasound suggests I am only between 4 and 5 weeks! How could I be a week so wrong?

My last period (albeit was also miscarriage bleeding) was 3rd August and I ovulated on 15th or 16th of Aug.... so surely I should be over 5 weeks??

The nurse says not to worry because of the miscarriage and previously having had the coil my dates will just be all over the place. But I'm just a little concerned!!

The scan showed the sac and the yolk sac and the nurses were happy everything seemed to be normal for a 4-5 week pregnancy. I have another appointment in 2 weeks time but this is just playing on my mind a bit!!

Any thoughts or previous experience?