The engines are required to survive a sudden loss of drive at contingency power during testing. This is done by running the engine on a test stand at the maximum contingency power rating and explosively severing the driveshaft. Tests (or prediction) somehow showed that the N2 wheel would disintegrate at about 150% N2 for the model of engine in the Bell 407. So the initial speed at which the overspeed protection would shut off fuel was set at 110%N2 in order to prevent the N2 from exploding. Later found to be too low (you could actually get the NR to 114-115% if you really worked hard at it in auotorotation. Thanks to the 'soft rejoin' feature of the fuel control, the N2 would run up to try to meet that value when you raised the collective to recover. Ensuing accident where the engine shut down at 110% N2 during a practice turning autorotation required the N2 overspeed value to be re-set at 116% if memory serves me well. Tests showed you couldn't get the NR there.
The original value had been chosen too conservatively without due regard to the big picture.
But the overspeed protection is required to demonstrate that it can survive an explosive immediate loss of drive at the worst condition.