Why doesn't 'w' or 'who' command show all logged in users?

Solution 1:

Desktop users won't normally show up in who because LightDM doesn't record your desktop login in utmp (see https://bugs.launchpad.net/lightdm/+bug/871070).

I wonder if, while you're testing this, you're leaving your terminal windows open:

1. You open an xterm and run who from your own account. You're the only user with a terminal, so it only lists you. The other accounts might be running sessions in the background, but they don't get listed because LightDM didn't record their login.
2. You leave the xterm open on your own account, switch to the kid's account, open an xterm there, and run who. Your account is still listed because it's connected to the xterm that you left running.
3. You leave that xterm open on your kid's account, switch to your wife's account, open another xterm and run who. Now all three accounts have terminals open, so who lists all of them.