Doctor changed my due date at 22 weeks pregnant

I am currently 22 weeks pregnant and I had my anatomy scan yesterday and they told me my baby girl was perfect and healthy and that she was weighing about 1 pound 3oz. they also told me that instead of my due date being December 18th(which is what I've been told all along) it is now December 14th they didn't explain to me why it changed all of a sudden but does it have something to do with her weight? Or did they just mistake the due date all the other times.