NAS CPU resources are shared throughout all of the applications running on the NAS. Every running virtual machine will be seen as a process in the NAS (for example, qemu-system-x86_64) and occupy the CPU for a given time. The CPU usage display of Virtualization Station depicts how much the virtual machine uses at that time.
When the CPU core allocation of a virtual machine is less than the total number of CPU cores, the CPU usage value will be different. For instance, if the NAS is only running a virtual machine (no other NAS applications running), the NAS CPU usage will be close to the one of the virtual machine.