More contributors is a sign of a good project though; it shouldn't be penalized for that and, if anything, the spread over contributors should reward a project as being better (often a sign of more different people using it, improved bus factor, distributed ownership/governance/approval, etc etc)
The way contribution rate is calculated is documented in the about page (http://www.krihelinator.xyz/about). I hope that it answers your question.
In general, it seems that just from reading the term "contribution rate" several people predict that it only count commits. Commits have the least weight in the calculation. The number of contributors are much more important as well as number of issues and pull requests.