Say a citizen owes some money to the government and he has make the payment to a govt official. Now if he pays in full, he has nothing to benefit apart from having cleared his due and the official would have performed his duty. Now say both of them come to an agreement and the citizen pays less than what he owes including the bribe to the official. Both of them are now better off individually, while the government loses what it had to get in full. We can also have a situation where one person defects. If the citizen does not feel the govt can do anything to get him pay his due, he does not and the govt official takes legal course and if the official feels citizen gets some some benefit by making his payment, he may demand more money including the bribe.