But do you have any idea what else was running on those virtual machines and what kind of overhead that gave ?

Yes, I know exactly what was running on those virtual machines. They were purpose built for this test, nothing else was creating load on those servers.

The virtual machines were not running anything else, but the machines they were running on might have been running something else.

Or are you sure that the machines these virtual machines were running on were otherwise completely idle ?

And if they were idle then why bother with the virtual machines, then you could have just run your tests directly bypassing the whole VM overhead.