Is the ultrasound wrong?


I had an ultrasound done a while back and I've just found out that according to that ultrasound I am a week less pregnant than I thought I was.

According to my last period and everything, I'd be 10 weeks 4 days now, but they just told me I'm supposedly 9 weeks today? Could I have got pregnant way after my period or is the ultrasound tech just off by a week?