Calculating due date

Brittany • Mommy to a wonderful 12yr old boy, 9yr old boy 7yr old girl 6yr old girl and 3yr old girl. Due March 11th 2023 with a boy.
They say they go by the 1st day of your last period. Mine was Nov. 10th my SO and I didn't have sex for the 1st time until the 22nd when we decided to try for a baby. So how can they go by my last period that's almost 2wks difference?