Based on lines changed, merge requests accepted/rejected, etc etc.
You know, GitHub developers are quite smart.
They could just write their own weighting algorithm if they wanted to. :-)
Those are absolutely awful metrics to use when measuring a developer's ability and talent, or the quality of this developer's contributions. They are also extremely easy to game, too.
Yes, that's why I trust GitHub to come up with a (proprietary, unreleased) algorithm a la PageRank that won't easily be game-able.