Now that's a good question.Sounds like they are saying GOD Is fair to some not ALL.
God being fair is tossing all mankind in an eternal hell. His mercy is that He sent His Son to redeem a multitude of sinners from their sins.
You're conflating fairness with mercy. No one wants God to be fair, trust me.
You kill someone, fairness would be you dying as you killed someone who is in God's image. Mercy would be them dropping the charges.