Good day! I don't understand, am I stupid or is it that my skis aren't…

Good day! I don't understand, am I stupid or is it that my skis aren't going? The gynecologist says my due date is 14-15 weeks, considering the first day of the last CD was August 19th, then it doesn't work out, and more. I know that the obstetric and embryonic term differs by 2-4 weeks. In the first screening, there was a difference of 2 weeks. Well then, 16-17 weeks, not 14-15.