I went past my due date. I was afraid in the beginning bc of the measurements - not knowing. There is so much imprecision in this measurement that it's laughable. It's to help give you a rough idea of whether the uterus and the baby are growing more or less properly without any sudden changes that might indicate a sudden change in the amount of amniotic fluid. During the first half of the pregnancy, most practitioners just note a crude approximation, i.e. fundus halfway between the pubic bone and umbilicus, or at umbilicus.
Babies do stretch out and curl up, after all.
A single centimeter is very, very small among all those factors. Most practitioners know the number of weeks before they measure, and it turns out that the measurement usually equals the number of weeks. Really, if it's off by much, you double check, assess fluid, and maybe make a note to check carefully in the future. But it's a very approximate measurement. I doubt that two practitioners at the same time or the same practitioner at two different times in the same day would get the same exact results.