Mmmm, this seems to be a common thing. When I was first scanned at 6wks 3 days (I had no idea how far I was, this is what the sonographer said) the computer she used to calculate dates said I was 5 weeks. But the sonographer over-rode it as she said it wasn't right.
The CRL was 3mm which I panicked at, as everyone said that was too small but I found a site somewhere that is like a guide for U/S techs. It said that the fetal pole grows at about 1mm every day from 6 weeks, so 3mm put me right at 6wks 3 days.
I've just had my second scan at what would have been 9 weeks 2 days based on what the sonographer said, and she was exactly right. I had been 6 wks 3 days at the first scan.
I'm not sure why but everyone who has an early scan seems to be getting told they're no where near as far along as they thought they were. Luckily my sonographer was experienced enough to recognise that the computer was wrong or else I would have gone out of my mind with worry.