Your due date can be off by a week or two since no one knows when u actually ovulated. My last pregnancy my son was meauring 2 weeks ahead of what my Dr said. Now with this little girl she's measuring 17 almost 18 weeks and I'm only 16 weeks 4 days according to my Dr. The sono is measuring her bigger. I wouldn't worry to much about it.
Mine said 11+4 an I was 11+1 and my OB had said my due date was the exact same. So I don't base it off that. Because if I did I'd be due August 5 not 18 haha big difference.
You probably ovulated early. Like when I went for mine I thought I was 12 weeks but I was only 10w 4d so I ovulated late