God is the Master of the Day of Judgement.
He is also Master of the period of reckoning and process of reckoning in this life. But in this life, human actions are often also judged and rewarded by other men – kings, rulers, superiors etc. – and there is always, therefore, the possibility of error.
On the Day of Judgement, however, the mastery of God will be exclusive and absolute, and the work of requital will lie entirely in His hands.
There will be no error, no undue punishments, nor undue rewards.
Unlike a judge who is bound to give his judgement strictly in accordance with a prescribed law, God, being the Master of the Day of Judgement, can forgive and show mercy wherever and in whatever manner he may like.