Hi HugeBlack,
Thanks for your interest! Below the answers.
You can't find bitcoin because that repo is the "Grandfather" and seems to don't have any parent repo.
The similarity is detected directly from the code using a pre-defined list of the repo (currently 62 repo).
The check is done by a script that we created ad-hoc, no human action affects the analysis.
All feedback or tips is very welcome.
Best Regards,