When using diagnostics to display the system information, like in the following example:

$ nova diagnostics ubuntu

+------------------+---------------+
| Property         | Value         |
+------------------+---------------+
| cpu0_time        | 1138410000000 |
| memory           | 524288        |
| memory-actual    | 524288        |
| memory-rss       | 591664        |
| vda_errors       | -1            |
| vda_read         | 334864384     |
| vda_read_req     | 13851         |
| vda_write        | 2985382912    |
| vda_write_req    | 177180        |
| vnet4_rx         | 45381339      |
| vnet4_rx_drop    | 0             |
| vnet4_rx_errors  | 0             |
| vnet4_rx_packets | 106426        |
| vnet4_tx         | 37513574      |
| vnet4_tx_drop    | 0             |
| vnet4_tx_errors  | 0             |
| vnet4_tx_packets | 162200        |
+------------------+---------------+

Why memory-rss could be greater than memory-actual? Based on my understanding, memory-actual should be the allocated memory to the guest and memory-rss should be the non-swapped memory the guest is using in the host server as a process. Anything wrong with my understanding? Or someone please help me by explaining this?