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Hi, Just because the Instance is shut off, doesnt release the hardware resource (at least not in Icehouse, Juno or Kilo) . The instance still has the hardware reserved, its just not using it. This allows you to have less usage but the ability to always boot the instance if needed. 

This is one of the reasons why you can “oversell” the resources with a CPU and RAM ratio.
I hope this helps.
Best regards,
//f