Anyone else know that you aren't as far along as they say?

I had my first ultrasound today. Originally, they had my due date at April 13th, making me 7w3d. But I track everything and I knew that wasn't correct. I figured when I got my first ultrasound they would adjust it by a week, making me only 6w3d. Or so. But today they told me I'm measuring 8w3d. I'm just curious how many other women have experienced your uterus measuring further along than you know you are. With my daughter they did the same thing. But not by two weeks!!