Host Demand isn't the sum of VM Demand : Why ?

Hello,

In the tab "operation - Detail" with a host selected, can someone explain me the differences between the "host demand" and the "Virtual Machines Demand" ?

I don't get why the sum of the VM demand isn't equal to the host demand (no even close !)

Here's a screenshot that illustrate my question : http://s30.postimg.org/5vkh4zx41/Host_vs_VM_Demand.jpg

Thank you.