Due dates, someone help

I’m having trouble wrapping my pregnant brains around this and need someone to explain it because my doctors didn’t.

At the start of this pregnancy, the doctor asked my dates and I only had one cycle to go off. And if I go by that date I’m due on the 28th of feb. but from the dating scan they are saying I’m not due until the 23rd of March.

It feels like a massive difference in expected arrival 🤷‍♀️

I’m confused!

Does your doctor go off scan or dates?