Competency in law involves more than the pure legal principles. If you want to be a good jurist, you have to know the law; you have to know the application of the law, but also have some sense of equity and justice. Principles of law are very general. They become tangible in their application, so you have to have good common sense and good judgment. It's more than just knowledge of the law.
If I go along with that idea of the sense of justice and knowing your society and people's values, all of that is important in the interpretation and the application of the law, because principles of law are very dry. They become alive when they are applied in a particular context. So it's a bigger knowledge of the law that you have to have.