Due dates are estimates. Nobody knows (aside from IVF or intense fertility treatments) what day you concieved. So Dr's are just doing their best to guess. There's clues - ultrasound, fundal height, last menstrual period - but those are just clues and guesses. Don't have your heart set on a "due" date because it's a best "guess" date.
Some women also have shorter pregnancies, some carry for longer. So you can't plan a date that your baby will be here. Baby cones when baby wants!
They are estimates.. I am 25 weeks measuring 30.. but the babies are measuring 3 weeks ahead..
All best guesses, babies come when they want anyway.