Why does it seem like my uploads are always disappearing?

I source a lot of my ereading material from KOBO.  I  always download the epub or DRM to my adobe digital library.  everytime I come back to the library many of the books I have downloaded are no longer there.  My current and only bookshelf has 280 items.  Some of the items on the shelf have a note that says 1 of 413 or 1 of 332.  I always assumed that this indicated "upload of 1 item to the list of 413.  WHich would mean that I have added 413 documents to the bookshelf.  So why does it seem like most of them have disappeared as the bookshelf currently says I only have 280 items in inventory?