Date confusion

So I got my first positive test on 8th December

We had sex 21st, 22nd and 25th November and also

1st December

Scan on monday 30th measured baby at 6 weeks and 2 days

Is that from ovulation/conception or predicted last period??

It works out that I could have ovulated around the 1st but how did I then get a positive test at only 7dpo??

I dont know when I ovulated and had miscarriage at end of October so dont have a last period to go off of 🤷‍♀️

6 weeks and 2 days doesnt match up to any of my dates so why would baby be measuring behind? We saw a heartbeat