Due date question

Shelby • Married to my best friend 4/15/17 ❤️ Milo 11/29/18 💙 Daphne 5/14/23 🩷

So according to LMP my due date is November 29 but I am measuring five days behind that which would put me at December 4th. My doctor is keeping me at the November 29 due date. Why would she want to do that?