Where do empty packages come from?

I'll admit, I'm a compulsive downloader, but I also try to be a safe one. I check everything through Clean Installer and get rid of anything that looks the least bit suspicious, and I have all of my downloads neatly organized into the proper folders, so that I know where everything is. However, I've been noticing lately then whenever I open Clean Installer to move things around and do general housekeeping, I'll find a whole stack of packages in the top downloads folder that are categorized as "Empty Packages" (and thus marked in pink in Clean Installer). The file names are always a string of senseless numbers and letters. I always simply remove these packages, but being curious I have to wonder why they are there. I thought maybe they were just corrupted packages, but none of the items that I've downloaded have seemed to turn up missing. So, where have these Empty Packages come from?

I'm not looking at this as a problem, I'm simply curious.