It's almost impossible to determine the exact moment of conception so in order to give you a due date doctors will go by the date of LMP but ultrasounds are much more accurate to determine baby's age, and measurements.
I have the same issue LMP is due date of Sep 14 at 23+1 but my 1st ultrasound was a due date of Sep 1st which makes my little man 25+1 day.. a two week difference which is driving me crazy lol ... But my midwife said the 1st ultrasound is the most accurate so you should deff. Check on that