How can `Get-Command` return "commands" that aren't really commands?

Question

'Get-Command' returns a list of all available commands in a powershell session.

The following code evaluates to $false, when passed a string which is not a command:

function f($x) {[bool](Get-Command $x -ErrorAction SilentlyContinue)}

And it does:

PS C:\Windows\system32> f ls
True
PS C:\Windows\system32> f alkdsjfasd
False

So you would think that the following code would return the list of commands which are not commands -- that is an empty list:

Get-Command | Where-Object {[bool](Get-Command $_ -ErrorAction SilentlyContinue) -eq $false}

Except it doesn't. It returns the following on a clean Windows Server 2016 VM:

CommandType     Name                                               Version    Source
-----------     ----                                               -------    ------
Function        Get-IseSnippet                                     1.0.0.0    ISE
Function        Import-IseSnippet                                  1.0.0.0    ISE
Function        New-IseSnippet                                     1.0.0.0    ISE
Function        Start-AutologgerConfig                             1.0.0.0    EventTracingManagement
Cmdlet          Add-ClusteriSCSITargetServerRole                   2.0.0.0    IscsiTarget

If I wait a little while and run the same command again, it returns a much longer list:

CommandType     Name                                               Version    Source
-----------     ----                                               -------    ------
Function        Add-RDServer                                       2.0.0.0    RemoteDesktop
Function        Add-RDSessionHost                                  2.0.0.0    RemoteDesktop
Function        Add-RDVirtualDesktopToCollection                   2.0.0.0    RemoteDesktop
Function        Disable-RDVirtualDesktopADMachineAccountReuse      2.0.0.0    RemoteDesktop
Function        Get-IseSnippet                                     1.0.0.0    ISE
...
Function        Test-RDVirtualDesktopADMachineAccountReuse         2.0.0.0    RemoteDesktop
Function        Update-RDVirtualDesktopCollection                  2.0.0.0    RemoteDesktop

I've checked the files in the RemoteDesktop module, for example, and I've been able to find the functions that are "missing".

What is going on here? How can 'Get-Command' return "commands" that aren't commands?