iui dating confusion

So my last Eric's was April 18, which would give me a due date of Jan 23. But I was going by my iui (conception date) which would make it Jan 24. I would think nothing is more accurate then the iui date which puts me at 7 weeks 1 day. But yesterday at m ultrasound they said I'm only 6 weeks 1 day. I asked if the baby is measuring small them and she said no totally healthy with strong heartbeat. Does this seem weird to anyone else?