Confused

I had blood work drawn on December 28th it was 404.

I had a ultrasound done and the sac measure 5 weeks..

Doctor said everything was fine during my exam and sonogram. I go back the 28th of this month. To early to see anything.

What I'm confused bout is my LMP was November 24th by Glow I'll be almost 7 weeks due date August 31st but by ultrasound I was 5 weeks on the 7th of January.