It could have to do with how the fetus is positioned. Also if you are short, it could lie that high. But generally, the top of the uterus can be felt at the bellybutton around wek 20, and then rises about 1 cm each week after that. It it rises to fast, you could have twins. If it rises to slow, there could be some problem with the fetus' growth.
But if your distance between the top of the pubic bone and the belly button is very short, then it could show higher even at 16 weeks.
Lol. Or you could have twins =8^0