The law can turn out to be different from what everyone thought. A higher court can reverse it. Even in the highest court, stating the definitive law, justices often dissent.

So the idea that the law actually exists, and we just have to find it, doesn't seem correct. Given that judges were usually barristers before, it's more like: one barrister confidently argues that the law has always been X, the opposing barrister confidently argies it's always been Y, the former-barrister confidently argues that it has always been Z - and that one always wins.