It's a question that comes up occasionally. Unfortunately it has to do with the way the program works -- it reserves the space so Windows will not let you put any icons into that space. (Not sure why it seemed to work differently when docked on the side).
It's useful for applications that draw on the screen, like Rainmeter, but not so useful if you want to put desktop icons into the reserved space.
What you COULD do, if you want to put icons to launch applications or documents in the reserved space, is try my Launchbar Commander
tool, which may be overkill for what you want, but does let you create (an optionally transparent) reserved docking area on your screen, and let you populate it with icons to launch files, shortcuts, menus, etc.