If you had sex on your (prime day) then I could see your question but sperm can live up to 5 days (I think somewhere close to there) so there is no way of knowing when you actually conceived that’s why they go off the first day of your last cycle unless your ultrasound is a huge difference.

It’s the first day of your last period, and it could change at your first ultrasound

They calculate it as 40 weeks from the first day of your last period which is 38 weeks from when you O (approximately)

That’s just how they measure it. It’s always 2 weeks different.