3rd trimester due date changed

I can't wait to meet my baby girl this will be second pregnancy for me I had my daughter at 36 weeks she was 3 weeks early and exactly 6 pounds anyway my doc told me at my last check up that I'm 34 weeks when I would actually be 35 weeks pregnant which is what I been going by and saying my due date went from Oct 2 to Oct 11th, but at all check ups my baby been weighing small as for her head size so they say and I'm thinking what does a weeks difference have to do with her weight and when I'm due.