Jesus says that two men go up to the temple to pray, one a Pharisee and one a tax collector. The natural assumption made by anyone hearing this story is that the Pharisee is the devout person. The tax collector, on the other hand, is the sinner.
Sure enough, the Pharisee steps away from the crowd in order to maintain his purity before God, and launches into a list of all his religious accomplishments: “God, I thank you that I am not like other people: thieves, rogues, adulterers, or even like this tax collector. I fast twice a week; I give a tenth of all my income.” He does everything right, according to the standards of the day, obeying all the religious rules of the road. In terms of keeping God’s commandments, he is way above average.
Then the tax collector bows his head, beats his breast, and says, “God, be merciful to me, a sinner!” He’s feeling so ashamed that he cannot even raise his hands and look up to heaven, which is the standard position for first-century prayer. The tax collector doesn’t make any boasts or excuses — he simply asks for God’s mercy.
There’s no reason to assume that this tax collector is a particularly spectacular sinner. If he were a thief, a rogue or an adulterer, Jesus would say so. It’s much more likely that he is confessing a set of secret, hidden faults — a collection of oversights, errors and miscalculations that only he would know.
So the above-average Pharisee boasts, while the sin-sick tax collector says, “My bad.”