CPU usage increase when changing number of cores of the VM lively

I am working on increasing the number of virtual cores of the KVM virtual machines lively using libvirt. I created a virtual machine with 2 cores. Here are the initial core settings.

virsh vcpucount 2
maximum      config        32
maximum      live          32
current      config         2
current      live           2

I also attach initial CPU usage output using htop.

After that, I increase the number of cores to 4 using the following command

virsh setvcpus 2 4 --live

I check the current core configuration of the VM

virsh vcpucount 2
maximum      config        32
maximum      live          32
current      config         2
current      live           4

It looks like I was able to increase the number of cores from 2 to 4. However, I realize weirdness in terms of CPU usage. After I allocated 2 more cores, I observed the following CPU usage. The problem here is that added two cores are at the 100% level. When I back to 2 cores again, CPU usage returns to normal.

Do you know why it happens?

Thank you in advance,