Why is this??

The main hospital that deals with most of my stuff still has my due date set for October 21st when it changed to October 15th at my 12 week dating scan. Even my regular doctors office has my due date for October 17th and not the 21st. I'm so confused as to why they haven't changed it to the correct date even though I tell them every single time I have an ultrasound (I'm high risk so I get ultrasounds very frequently). I'm not worried about my doctors office having my due date pushed back by only two days but the main hospital has it pushed back 6 days..