Memory usage is inconsistent between nova and the actual used RAM on the hypervisor

Issue

• When logging on the hypevisor, free reports a given amount of RAM being used:

[heat-admin@overcloud-compute-1 ~]$ free -h
              total        used        free      shared  buff/cache   available
Mem:           503G        396G         97G        5.2M        9.0G        100G
Swap:          256G         48G        208G

• When using the OpenStack CLI, a different value is reported:

# openstack hypervisor show overcloud-compute-1 | egrep "memory_mb|free_ram|running-vms"
| free_ram_mb          | 193314  |
| memory_mb            | 524066 |
| memory_mb_used       | 330752  |
| running_vms          | 20  |

• The above is true in Horizon too when the hypervisors statistics are displayed as per this screenshot: