Project Contributors chart shows larger commit count than repository API

Summary

The project "Contributors" page (/:group_slug/:project_slug/-/graphs/master) is showing significantly different numbers for "commits" than what is returned by the repository contributors API endpoint. This bug was originally reported in #233119 (comment 392517703).

Steps to reproduce

• Go to https://gitlab.com/inkscape/inkscape/-/graphs/master note the number of commits displayed for any specific author.
• Go to https://gitlab.com/api/v4/projects/inkscape%2Finkscape/repository/contributors?order_by=commits&sort=desc and note the number of commits displayed for that same author.
• See that the UI numbers are much higher than the API numbers.

Example Project

https://gitlab.com/inkscape/inkscape

What is the current bug behavior?

The number of commits displayed in the UI is significantly different from the number of commits returned by the API.

What is the expected correct behavior?

The number of commits is the same in the UI and the API.

Output of checks

This bug happens on GitLab.com.

Possible fixes

(If you can, link to the line of code that might be responsible for the problem)