This little known fact came up already twice recently so I will write just a short post about it.
In the past when you created allocation pool Organization Virtual Datacenter (Org VDC) and allocated to it for example 20 GB RAM it actually did not allow you to deploy VMs with total memory sum of 20 GB due to virtualization memory overhead being charged against the allocation as well. This behavior forced service providers to add additional 5%-15% to the memory Org VDC allocation. This was also very confusing for the end users who were complaining why their VM cannot power on.
With the elastic allocation pool VDC changes which came with vCloud Director 5.1 it is no longer an issue. The reason is that in the non-elastic VDC it is vSphere (and Org VDC resource pool object with a limit set) who does the admission control – i.e. how many VMs can be deployed into it. In the elastic VDC it is actually vCloud Director who is responsible for the decision if a VM can be deployed into particular VDC.
So to sum it up: if you use elastic allocation pool the tenant can use it up to the last MB. The virtualization VM memory overhead is charged to the provider who must take it into account when doing capacity management.