I suspect this is just a refresh issue with the ArcToolbox window. There is definitely only one item description that you edit using the item description editor, available in ArcCatalog or the Catalog tree in ArcMap.
After editing the Item Description in Catalog, exit the application (ArcMap/ArcCatalog). Open the application, show the ArcToolbox window, and see if your item description has been refreshed. If that doesn't work, try removing and adding your tool to the ArcToolbox window. If neither of these work, I'll do some more investigating.
What version you running? Obviously, it has to be 10.x since Item Description was introduced at 10.0, but what specific version you on?