It's a case of the criminal's dilemma. Two people do a crime. If both refuse to talk, sometimes both will walk. Maybe the evidence is not so strong and two trials are costly. So the prosecutor selects the weaker less guilty party and offers a deal to implicate the other party at full sentencing.

In this case a deal was made and everybody pretended it did not exist. How this could go through a pre-sentence hearing and not be brought out requires negligence, stupidity or criminal behavior by the prosecutor.