Man interprets justness, or more precisely "justice" as a balancing of the scales. God's just-ness resides in the immutability of his word.
To paraphrase Dr. RC Sproul in his book "What is Reformed Theology": There is a famous adage that adorns the bumpers and rear windows of many a Christian's car: "God said it. I believe it. That settles it." While this phrase may bring a smile to our face, it is in fact, very humanist in nature. Whether the driver believes what God says has no impact on the "settling" of the matter. The phrase "God said it. That settles it." is the universal truth of God's justice.
Balancing of the scales plays no role in God's justness, to define it by man's interpretation is to impute malleability to God's immutable law. For example, homicide is segregated into many different categories by man from aggravated murder to abortion, from capital punishment to "justifiable" homicide. The worldview uses the programming if..then...unless...
For example, If Abe strikes Ben, then Abe receives punishment unless it is determined that Ben struck Abe first, then Abe should receive less punishment than Ben because Abe was defending himself; or it is determines that Abe unintentionally struck Ben then of course it was a mistake and Abe's punishment may be limited to apologizing to Ben or paying restitution to Ben for recklessness.
God is perfectly just, in that, the unless is omitted entirely from the equation. The demarcation of God's laws is absolute, it's an "either/or" statement with inalterable consequences. To take the example from above, if God has determined that striking another person is against His law, once the determination is made that Abe struck Ben, then, regardless of the circumstances, Abe must face the consequences of his action.
Depending on the circumstances, humans may perceive the consequences as unjust or as justice, but the rest of the story is this: God created the demarcation between right and wrong, good and evil, order and chaos as well as the consequences prior to the creation of man and therefore was not influenced by His love for His creation. His perfect law was set forever at the moment that He created order within the void. Therefore though the consequences may seem harsh to our eyes, we fail to see that they are perfectly just because of the very fact that they are unmoved by circumstances or further evidence.
You are either on the right side of the line or the wrong side of the line, that is not the fault of the line nor is it reason to move the line. And God does not obscure the line, one of the wonderful manifestations of God's all encompassing love for us is that He does not play "gotcha", He is very clear in both His word and through natural revelation where the lines are drawn.
The ten commandments are a perfect example of this clarity, take the 6th commandment: "Thou shalt not kill"(KJV) or "You shall not murder"(NIV)(Exodus 20:13). This is a clear demarcation of right and wrong action leading to just consequences. If you kill or murder another human being, you have earned God perfectly just consequences. Regardless of the circumstances the taking of the life of another human being is against God's law and the consequences for crossing that line are the same for the serial killer as for the doctor that administers the lethal injection for the serial killer. That is a challenge to a worldview which justifies wrong action with circumstances, but, again, God is perfectly just and therefore impartial, because if His justness could be influenced by our persuasion, works or restitution then His grace would have no value.
God's judgment is impartially administered based on His immutable law. And it is because of this that our joy in Christ's work should reach new heights every single day. Because nowhere in creation is the fact that God is just more vividly displayed than through Christ's justification of us to God which leads to eternal life. But let's not get too far ahead of ourselves.
The human experience with the justness of God began in the Garden of Eden.