Low pappA can also be a sign of placenta not working so good we had a bad NT and the BT we were given a 1 in 2 chance of something wrong the doctors told us all senerio's and one of them was a not so great placenta can cause the low blood levels in our case it was bub who had a genetic condition but as your U/S was normal that is a good sign it might just be the placenta and what the doctor said to me it means it could effects bubs growth so they have to keep an eye on it and hope it rectifies itself or if not they take bub a bit early.
I hope yours is just the levels were a bit off.