Here is the ‘short cache description’ information as I found it and as it still exists directly on the geocache.com website:
http://www.geocaching.com/about/cache_types.aspx
"A letterbox is another form of treasure hunting using clues instead of coordinates. In some cases, however, a letterbox has coordinates, and the owner has made it a letterbox and a geocache. To read more about letterboxing, visit the Letterboxing North America web site.
That is where I gathered my interpretation from. When I read ‘In some cases…’ it led me to believe that we were now discussing the letterbox hybrid?