There are, of course, ideal measurements depending on the age of the foetus. At 21 weeks of age, the fundal height should be between 19 and 23 cm. A fundal height that is bigger than expected may mean that the baby is a bit bigger than normal or there are multiple foetuses. A fundal height that is too small simply means the child is smaller than average.
If the fundal height is not consistent, this may prompt your doctor to order an ultrasound to determine the cause. After 36 weeks, the fundal height ceases to be useful as it is often inaccurate because the baby has moved position to get ready for the birth.
Really fundal height is just one of the tools a doctor uses to help ensure the baby is keeping on track and is healthy. A less than average height does not mean that anything is wrong with the child, only that maybe it is worth doing some more tests just to be cautious.