Why doesn't the number of commits for a project match the sum of commits from contributors?

Question

This project shows 1993 commits, but clicking on Contributors shows a total less than half of that, only 925 total.

Why the discrepancy? Each branch has over 1800 commits.

Answer 1

The Repository Graphs help page mentions:

The Contributors graph shows commits made by the top 100 contributors over the lifetime of your project.
This graph doesn't include merge commits or commits with zero changes.

If there are many PR for this project, that could explain the delta (or part of it anyway)