Dates??

So today I thought I was 8+4. I just had my follow up appointment from my ultrasound and was told I’m exactly 9 weeks today. I know the doc said it doesn’t really matter as it’s only a few days but I’m really confused. To my calculations my LMP was the 13th of July so it doesn’t really add up if this is correct. Now I’m second guessing when my LMP was 🤨