[NEST Users] V_th Issue in Izhikevich Neuron Model

Dear Nest Community, When trying to reproduce Izhikevich's Simple Neuron Model with NEST Izhikevich Neuron Model, it is spiking before reaching my V_th potential threshold setting of 30 mV, producing results inconsistent with the publication. Would anybody have insight as to why the neuron is spiking early? Did I perhaps not include a relevant NEST setting? I compared it to an explicit implementation of the ODEs, attached, for your reference of expected neuron response. Thank you for any suggestions. Best Regards, --Allen