Earlier than expected?

So I went in to have an early ultrasound (I had a previous ruptured ectopic with a tube lost so they get me in early to confirm location). I thought I should be about 6 weeks, but the ultrasound tech said they didn’t see anything except a thickening in my uterus and a small dot that could be the gestational sac but he thought I was way earlier than 6 weeks. My hcg came back at 478 and am going again tomorrow for recheck... anyone have experience being off on dates?! I am wondering if it was another miscarriage and things are just shrinking down now after it. I will obviously know more with my numbers tomorrow but just wanted to see anybody had that where dates where off and still was a viable pregnancy.