When you installed TW3 pre-load using the Client, it downloaded to your computer in the installed form, ready to update and play.

In the Extra Downloads folder you can also download goodies as well as standalone multi-part installer of what you already downloaded (it's the same multi archive thing as available on the website).

It works this way for every game - you can either install it with one click or download backup for archiving needs. So it's not the glitch, just unfortunate naming.