The time of pregnancy is calculated from the first day of your last menstrual period (LMP), which is typically two weeks before conception actually occurs. This means the first two weeks of the 40-week count are before fertilization, during which your body prepares for ovulation and a new cycle begins
The due date is just a guide. In,y abou 10% of births are on the due date.
So? It’s not like pregnant women are planning weeks in advance for an exact date to give birth. An estimated due date is exactly that: an estimate. I don’t have exact statistics on hand but if I remember correctly, your 10% are even a bit high and it’s more like 3% on the exact date. But about 50% are within +/- one week of the original due date and 80% are within +/- two weeks which is pretty good accuracy for a 40 week time span¹. If you adjust based on ultrasound results, you can get even more accurate estimates but the original due date gives you a good timeline when those ultrasounds (and other examinations) should be done.
¹ seriously, try estimating any other 40 week project to within a week with 50% accuracy.