One straightforward way to see that this is not so starts from the realization that the criminal law cannot perfectly anticipate all realistically possible cases. This is so for at least three reasons. First, human powers of anticipation are limited. Second, a criminal law doing justice to all realistically possible cases would be too complex for citizens and officials to comprehend. Third, such a criminal law would also be impossible to administer fairly because it would give criminals too many opportunities to escape punishment. Example: recognizing a rare, morally valid excuse in the law might be a bad idea if the absence of this excuse is very difficult to prove beyond a reasonable doubt.
We can expect, then, that in some cases punishments warranted under even the best-designed and -administered criminal law are excessive. In those cases, at least, mercy would not be unjust. For example, we may pardon an offender because we are convinced that he has a morally valid excuse that the law, for good reasons, fails to recognize (i.e., does not allow judges and juries to take into account).