It fits like lego really.
If God is just to those who are just, then that is perfect justice and mercy: the unjust are corrected. Totality = justice.
If God treats the good exactly as the bad, then He is being unjust to the good. Totality = injustice.
Another way to look at it: if you jump off a tall building onto rocks, God can either bend his law to you OR let nature take its course. The prior is not just because it's not consistent in itself. The way of God does not change. It's a river that always flows downstream to enrich the Earth (mercy/grace) and always forwards in time (justice). If someone tries to flow upstream then they themselves are being unjust and unmerciful. If they swam with the river then this would not even matter!
Maybe you've heard of the
Tao Te Ching?
Yet still, all beings other than God will always be less just and less merciful than God. Creation is limited and has desires whereas God is free of limits and free of desires. God is
the truth itself, the only correct reality. Being unquestionable, God is always correct. Mathematically speaking it's like Godel's incompleteness theorem. If you find out why 2+2=4 then you cannot find out why the proof to
why 2+2=4 is correct. Trace this recursion to the end and there is always
the unquestionable - also known as God. So the answer to your question is: because.
It's a matter of the identity of God. The error in reasoning comes from treating God and
truth or
reality as distinct things. "Is God true?" or "Does God exist?". Questioning veiled tautologies, you see. All of these terms are synonyms.