Sorry If I wasn't clear enough but what I meant is this:
Definition of resource pool taken from msdn website is, it represents a subset of physical resources available to DB Engine instance.
That means it's a subset of the resources you defined by setting the max memory for the instance (plus some other settings like CPU affinity.)
When I wrote that's not the same as the physical memory, I actually meant the available RAM in the machine.
Let's say you have a server with 32Gb of RAM and you set the max memory for the instance to 25GB, a 20% RAM limit in Resource Governor would be 5 Gb