I use nmon on a few vServers to watch the RAM usage. They all have Debian 8 or 9.

Everywhere nmon shows "logical" data as Total Memory (512, 1024, 2048, ...) but one hoster only give me 491,4 MB RAM.
My configuration needs 510 MB minimum to run without problems so i asked the hoster to give me the missing 20 MB because i pay for 512 MB and dont want to take the more expensive tarif.

After they wanted a screenshot and then showed me my tarif plan (where they wrote 512 MB) they now try to tell me its the OS which use the missing 20 MB.

Can someone please confirm that Total Memory is the memory of my whole machine without OS or anything else?
Maybe even with a short explanation how nmon measure the Total RAM?
Thanks