I'm not an S-61 qualified pilot so I write the following generalisation from experience on a number of other twins; hopefully with some relevance!
In the event of OEI, the Nr is drooped to that figure with collective to ensure you have pulled to maximum OEI power on the remaining engine (be it 30 second, 2 minute, or whatever power your type of engine is designed to give in that situation).
That sort of droop figure will bring the blades to their most efficient angle of attack at takeoff/ landing flight speeds.
If you droop lower the torque will begin to increase with no beneficial effect because you are going "behind the drag curve" on the main rotor.
Also, of course at the same time the tail rotor is slowing down too, which bring you closer to a LTE situation.