Files changed on GitHub differs from the result of git diff master

What is going on here?

GitHub pull requests use git diff master..., which compares the tip of current with its branch point from master.

That is not the same as git diff master, which compares the tip of current with the tip of master.

The official docs have a precise summary of the comparison types.

So, run git diff master... locally to see the same output that GitHub shows in its pull requests.