("does not prove they are" in the last sentence means "does not prove that an ultrasound done later in the pregnancy was accurate for dating conception.") If your ultrasound was in your 6th or 7th week, it will be reliable for producing an estimated due date and estimated conception date. But if it was later than that, it might not be.
September 27, if your daughter was born at 40 weeks exactly. Did you get your due date from an early ultrasound? Later ultrasounds might not be very precise for giving a due date, and even having the baby near the estimated due date does not prove they are if the baby was early or late.