I noticed that my VirtualBox VM typical CPU usage does not match the CPU usage in host.
I.e. when I import a MySQL dump, which is very much a single thread load, most of the time 2 CPU cores in host are utilized. When the VM is idle, then host CPU is also idle.
VM OS: Alma Linux 8
Host OS: Windows 10
What is that additional CPU usage is coming from?
Virtualbox doesn't set aside host cores for the VM to use, or keep the VM's code operating only on the particular number of cores set in the VM's settings.
Instead, Virtualbox sets up the correct number of processes to handle the number of "cores" it will present to the VM's OS. Then Virtualbox hands all these processes to the host, for the host OS's CPU scheduler to figure out which host core to run it on or spread around as it determines.
So it is not unusual to see a X-core VM using Y cores on the host.