Purpose
PDQ Inventory is not showing all Windows Updates that are shown in Control Panel > Windows Updates for the computer.
Resolution
Make sure the computer has been successfully scanned recently.
PDQ Inventory uses a WMI query to detect which updates are installed, this will only show Windows OS updates installed by Windows Updates or MSU files. It will not show other program updates such as Office, Adobe Reader, etc, that may show in the Control Panel.
Get-WmiObject Win32_QuickFixEngineering
When you install a Windows Cumulative Update, it includes all the hotfixes and changes from all previous updates, so you will only see the single KB number for the Cumulative Update. This means the computers could be fully up to date, but only list a single update KB number as being installed.
See Also:
WMI QuickFixEngineering Documentation