Someone told me that the bottom of my uterus is smaller than it should…

Someone told me that the bottom of my uterus is smaller than it should be at a certain stage. I had an appointment, and the doctor measured everything, saying it's 24 cm, too little for 30 weeks. I didn't even know why they were measuring it. On ultrasound, everything is fine, and the baby corresponds to 31 weeks. Now I'm curious, why could this be? There's no anxiety at all, according to the ultrasound, the baby is developing well.