confused on first ultrasound help!

Christina
Ok so on nov 29th I got my first ultrasound I was 6weeks and 0days. So I counted back 6 weeks from the 29th it put me at October 25th. But why are people telling me I didn't conceive until nov 1st? Wouldn't I of got pregnant a few days before October 25th ? 
 f that's true then how was I 6 weeks on nov 29th. I'm about 10 weeks an 6 days now.