When MongoDB asks for a memory address, the operating system will return a virtual address. This may or may not be an actual address in the RAM, depending on where the data resides. MongoDB will use this virtual address to request the underlying data. When we have journaling enabled (which should be almost always), MongoDB will keep another address on record for the journaled data. The virtual memory refers to the size of all of the data requested by MongoDB, including the journaling.
The mapped memory excludes journaling references.
What all of this means is that over time, our mapped memory will be roughly equal to our working set, and the virtual memory will be around twice the amount of our mapped memory.