It probably uses the remaining memory too, but not for memory allocations of programs but the kernel in the VM for page caching, i.e., as this is a NAS and thus has probably some read/write ops going on the page cache is surely a good use to buffer that.
So, as this can be made available for programs inside the VM it does not shows it as "used", but the PVE host sees only that the memory is in use.
You could execute the following command
free -h
in a shell in the VM, it should output something like:
Code:
total used free shared buff/cache available
Mem: 62Gi 41Gi 6.2Gi 1.0Gi 14Gi 19Gi
Swap: 30Gi 0.0Ki 30Gi
I.e., used by programs is 41Gi, the kernel uses 14 Gi for cache (unused memory is wasted memory), but that counts into the available of 19 G, because if a program requires more memory the kernel can simply drop caches to get it available (IO performance can then get worse, naturally)