virtual machine – Why does VirtualBox show more CPUs than available?

In my case, using 6 out of 8 CPUs in a 4 core 8 thread processor was faster in tests than 4 out of 8, despite VirtualBox cautioning that I may get degradation.
So I think that VirtualBox prevents one from shooting oneself in the foot if one is a newbie and will complain about slowness, while as a superuser you may dismiss this caution.

I have Intel-Core-i7-2820QM with 4 cores and 8 threads. I have Virtual Box 5.2.6 with Windows 10 host and guest.

Setting 6 CPUs out 8 in Virtual Box Guest settings gives next values in userbenchmark:

Multi core

MC Int 307
MC Float 326
MC Mixed 315
47% 316 Pts


Quad core

QC Int 270
QC Float 272
QC Mixed 268
61% 270 Pts

CPU-Z:

Setting 4 CPUs:

Userbenchmark

Multi core

MC Int 260
MC Float 269
MC Mixed 269
40% 266 Pts

Quad core

QC Int 260
QC Float 264
QC Mixed 262
59% 262 Pts

CPU-Z:

Notes:

No thermal throttling (which happens when 100 degree Celsius) happened during test runs: