Probably because for historical reasons, my repo is not the "root" on GitHub (it started out an an anonymous snippet on EmacsWiki and someone else put it in its own GitHub repo, which I later forked).
It looks like that's because its using GitHub's timeline API, which -- in addition to GitHub's general repository API -- only retrieves repositories which are actually rooted at the given user. Sort of a shame; I feel like most people's most interesting/important open source activity is on repositories which they don't own.
At the same time, this probably cuts down on "me too" contributions. I have a repo that has been gaining popularity lately and have been flooded with pull requests that do nothing but add a jshint file or other dev environment changes.
Probably because for historical reasons, my repo is not the "root" on GitHub (it started out an an anonymous snippet on EmacsWiki and someone else put it in its own GitHub repo, which I later forked).