confused 😕

Issy

Yesterday I went for a scan and the results read: 33weeks and 2 days and my due date is on 15 September 2018. I had my last periods on 6/12/2017. This app calculated my due date basing on the inf I gave and my due date is on 13 Sept 2018. (that is the same date my midwife gave me on my first appointment). I'm so confused how is this possible, which one should I follow?