The big difference is how it is organized to the users viewing it. My GitHub account is littered with old forks of repositories I created just to submit one line patches; and despite the pull request to a repository in some sense being data relating to the history of that project -- certainly the discussion is all organized under that project -- if the original person actually removes fork to garbage collect their namespace the commits referenced in that pull request just disappear. Meanwhile, despite people now using the word "fork" for this purpose due to GitHub, there is actual value in being able to search for actual forks of a project--things that people are choosing to publicly distribute and maybe maintain themselves--rather than seeing a thousand repositories which exist only for the purpose of contributing a single patch (or, though this is another topic, people making the metaphorical equivalent of a "backup copy" within the strange set of semantics and ownership that is GitHub).