GitHub's "Contributors" graphs are unreliable for working out who really works on a given repo. They (sneakily) only attempt to visualize the commits made by GitHub's own users. GitHub, however, is of course not the whole world. (Gimp itself does not even coordinate development through GitHub.) Commits not tied to a GitHub account are not represented.

Just another lame example of how GitHub both (a) insists upon itself and (b) prioritizes its social network over providing actually useful tooling for getting work done (including but not limited to trying to glean insights about a given project).