Proxmox is not able to see the Disk Usage of an VM, this might be possible with the QEMU Agent but currently technically not possible. VMs are a full virtualization, so the node can not look into the VM. If you using CT then it is possible because it is not an Full virtualization.
With Memory it's the same, so every OS has its own Memory Management. Windows itself consumes most of the RAM for Caching before it use the swap File in the disk. For the Node it looks like the whole memory is in use but the OS running in the VM using it not complete or can free it up immediately when its needed.
You can install and activate the QEMU Guest Agent and see what happens with the Usage, this might be give you an cleaner view of the Memory Usage because the Node Can now see whats going on in the VM.