Due date?

Hi everyone, I have a question about due date. I know it's not off that much but I was just curious. We did <a href="https://glowing.com/glow-fertility-program">IUI</a> to get pregnant and our due date based on the <a href="https://glowing.com/glow-fertility-program">IUI</a> which was done on 11/22, the <a href="https://glowing.com/glow-fertility-program">IUI</a> happened on 3/02. We went in for our first ultrasound and the baby was measuring with a due date of 11/27, which kind of scared me since we did <a href="https://glowing.com/glow-fertility-program">IUI</a> but they said not to worry. Then we went in for our first OB exam and they said our due date was 11/24, why would they pick a date in between the two when the baby never measured for a due date of 11/24?