Allocation Pool memory usage calculation

I had created around 10 VMs using allocation pool model and the total memory allocation is 31GB.  All VMs were turned on and idle with OS installed only.  However, the total memory usage shown in vCloud Director is 44.17GB.  Does that means there's an overhead calculation in vCloud Director?  If I change the memory allocation to 31GB, then I won't be able to turn on all VM because of insufficient memory resources.  How do I calculate this total usage?

Thanks for answering.