My figures exclude shared memory and are calculated as VmRss - Shared from /proc/<pid>/statm.

If you are using Windows 10, your missing Atom processes will be under Background Processes in Task Manager. For the sake of the argument, I just did a fresh install of Atom and this is what I see on the first run: https://i.imgur.com/0ZRSumF.png. ~220MiB (no files open, zero extensions).