I'm sure if the doctor is not concern there should be no worries. It's better closed than slowly closing. By that age it should be almost closed.
All I know is that a baby has a soft spot on their head [a hole in the skull]and they can die if an injury occurs. If the spot closes early then the baby is at less risk of those unwanted injuries.
All that means is that the soft spot in her skull which has been there since she was born, is hardening at a fast rate. Every child's soft spot becomes hard at a different rate. As long as you and the doctor do not see any significant changes in your daughter, and he/she is unconcerned, don't worry about it. It's perfetly normal.