Due date confusion!!

I have been to the doctors today to get the ball rolling with regards to midwife appointments. I am 5 weeks and 4 days according to all my apps, my last period started on 11th nov 2017 so my due date was calculated at 18th August. Even when I looked on the nhs (uk hospitals) website for due date calculator it came back with the same answer. However my doctor has put my due date at 26th September! I’m very confused as this would mean I’m technically not pregnant yet surely!