Usually it's just that your baby is bigger than average. They normally won't change any dates unless it's a drastic measurement lol
For most of my pregnancy I was measuring a week ahead, and then once I hit 34 or 35 weeks I started measuring a week behind. But my doctor won't change the date or anything. My conception date has stayed May 19th my whole pregnancy. It could also be human error causing the measuring to be ahead or behind, because when the doctor measures you, they just use a tape measure and feel what the believe is the top of your uterus from the pelvic bone.
Conception date does not change. It just means your baby is growing quickly. When you conceived is when you conceived point blank.
I was wondering the same thing!