Your charting (or git more likely) doesn't seem to take into account fast forward merges. This is evident by the lack of the sub-arch trees for PowerPC. There are at least 3 trees that commits flow into before they go into benh's tree, and none of those are present in your chart. Those sub-arch maintainers try to make it as easy as possible for benh to merge things, so the pull requests are often simple fast forwards on top of his tree. I doubt you'd get a merge commit there.