The AS-350B-2 is powered by an Arriel 1D1. I believe that this is and Ng limited power plant. As such the Ng limit is calculated as Ng divided by the square root of Ø, where θ represents the ratio of the air stream temperature at a chosen reference station relative to sea level standard atmospheric conditions.
The jog in the chart allows for the increased Ng capabilities of the engine at higher ambient temperatures. This explanation comes with the disclaimer that I had to dig far deeper into the bowls of experience and past training than would be considered reasonable.