Whoever judges applies a law to a single case. Here there are two different questions.

The first question:

What happened? Is there a contract? Did someone suffer an injury? Who was the doer? In criminal law, there is one more question: What kind of deed was it? Was it a crime, and which one?

The second question:

Which law fits this case? How do we restore right? In criminal law, this means: What punishment follows?

These are two different tasks. So different bodies can decide on them.

Two examples:

In ancient Rome, people kept the two apart. The praetor said: If the matter stands like this, then this holds. And a separate judge checked whether the matter really stood like that.

In England, the prosecutor decides what kind of crime it is. For example, whether it was murder or manslaughter. The court cannot change this, even if it thinks this label is wrong.

Summary: In court there are two questions: What happened, and which law fits it? These are different tasks. Different bodies can decide on them.