^ Ah but fundal height measurements are far from an exact science. Many obstetricians (mine included) don't take them at all because their accuracy is so questionable and they're entirely subjective - measurements can vary hugely depending on the person doing the measurements. I've never had my fundal height measured in either of my pregnancies.
But ultimately if the baby is fine sizewise and ultrasound has ruled out excessive fluid there really can't be too many other causes so I wouldn't worry about it too much.