The S12518 has 32 CPUs. It appears that the utilization of each CPU is added when reporting overall CPU utilization, either by NPM or within the switch code. For example, if each of the 32 CPUs is running at 1% utilization, NPM would display 32% CPU utilization. I suppose (but have not reached these levels yet) that 10% utilization of each of the 32 CPUs would result in a 320% reported overall utilization.
I can't seem to find whether this is a known issue or not, and how to get it addressed. Any suggestions?