Due dates

So, I’m a bit confused. Based off of the first day of my last period, Nov. 21st. I would be close to 8 weeks a long. But I know I ovulated around December 3rd-5th. I just had my first ultrasound and it confirmed that i’m only 6 weeks along. But the doctors are acting a little confused and saying “this can be normal if your periods weren’t regular” which they were always regular. So i’m just a little confused. Why would the first day of my period be considered the first day of pregnancy when I’m bleeding, on my period, and haven’t ovulated yet? Can anyone explain?