In photoshop, 100% relates to pixels, not physical (printed) size. Thus, when loooking at a document at 100%, each pixel corresponds to one pixel on your screen.
When translating this to physical size, it gets complicated as all monitors are different resolutions. It won't necessarily work to be 33.3% - it depends on the document and your screen resolution. But it's not a cop-out on their part - when editing, it's more useful to know that you are looking at all of the pixels than how big it will print.