I just love how Calvinists like to dodge the answer. Everyone deserves justice, but some were given mercy, for no apparent reason, at least no reason that any Calvinist will admit to.
But, in this system, God is clearly not fair. But that never bothers Calvinists. They don't care, as long as they are secure in the knowledge that they have been chosen.
Yet, God is as fair as He is just and merciful. Everyone is able to receive His free gift of eternal life. Those that do not receive it through faith do not have eternal life and are not qualified to live with God in eternity. So the only other residence for them is the lake of fire. That's the only reason they are there, since Jesus Christ paid for ALL the sins of ALL of humanity.
So, while the action of faith does not earn mercy or grace, God decided on His own to extend grace and mercy to those who believe.