OOB operational status for Virtual machine instance class

Can someone point me to the documentation that states how Operational Status is driven on the Virtual Machine instance class (OOTB)?  

We have VMs with State = Terminated and the Operational Status is showing as Operational for most and Non-operational for a handful.

We also VMs with State = Off and the Operational Status is showing between Operational and Non-operational. I haven't been able to identify a pattern.. we haven't done any changes to OOB CI Lifecycle management to date so why are these different by default in so many cases..? 

I don't see an obvious pattern.. Appreciate the help in advance.