They are following the /key/value/key/value pattern, but the first two pairs in a GitHub URL are fixed to user and project, which lets them omit the key names. I could see them not being willing to hardcode the third pair to blob.

Back when GitHub URLs were kind of cool, github.com/user/gritzko/project/beagle would have been much less cool than just github.com/gritzko/beagle.