Different due date according to ultrasound

Haley • Wife*Teacher*Mommy of 2 Rhetlynn 2/10/20* Ripken 6/30/21*Great Dane mommy of 2*

My lmp was dec 14th which put me at 6 wks 2 days going to my first ultrasound. According to my LMP, my due date is 9/20. When I had my ultrasound , it has my gestational age at 5 weeks 1 day and my due date 9/27. What does this mean? What one should I go with? My cycle was regular and was 26-29 days.