Are the commits from libav merged or cherry-picked/rebased? In any case, I would hope the stats were gathered with --no-merges; if not, that would indeed skew the stats.
Libav:
$ git shortlog -ns --no-merges v11..HEAD | head -n15
FFmpeg:
$ git shortlog -ns --no-merges n2.4..HEAD | head -n15
[1] https://lwn.net/Articles/650830/