Could it be that she gets the math conceptually, but hasn't had experience actually solving problems? DSs tester mentioned to us that he grasped above level concepts and could set problems up, but wasn't always able to complete the calculation. She felt it was due to not having the instruction yet. I do see that DS can do mental math far easier and quicker than he can do it with pencil and paper. Once he writes it down, he gets it all confused, especially if he's not had practice with it.
Just a thought.
