Plugins not identified as already installed

From time to time there are plugins that show up in the available list that are already installed EVEN THOUGH they are already installed AND the 'Get More Plugins' options are set to only show uninstalled plugins.

If you elect to install a plug in you are not sure is actually installed, the installer recognizes this and says something to the effect "appears to be already installed, forcing reinstall"

What sets the already installed plugins flag to be omitted from the list?