This GHTorrent website warns in its FAQ for reliability issues with the dataset. (See The Promises and Perils of Mining GitHub, Eirini Kalliamvakou et al, 2014 http://gousios.gr/bibliography/KGBSGD15.html )

For this project specifically: VI. Only a fraction of projects use pull requests. And of those that use them, their use is very skewed. VII. If the commits in a pull-request are reworked (in response to comments) GitHub records only the commits that are the result of the peer-review, not the original commits. VIII. Most pull requests appear as non-merged even if they are actually merged.

Is this advice ignored or were actions were taken to mitigate the documented reliability issues with the GHTorrent source data?