The actual images aren't quite what I would call "hi-res" - 512x512px at 72dpi for the Octocat, for example. Seems like if they are going to all this trouble they should include a few sizes, not just a few formats...
Specifying the dpi gives the print size of the image. So basically instead of printing very small and hi-res this will print as larger but with low-res; so it makes sense as the claim is that the images are not hi-res.
