I’m not sure that GitHub uses CommonMark! Generally GitHub's format is referred to as GFM (GitHub-Flavored Markdown) and it has notable differences including preserving newlines.

Notably, GitHubbers and StackOverflow representatives joined to define CommonMark, but GitHub and StackOverflow do not have remotely similar fenced code block conventions.

Also, CommonMark currently says that they have not yet defined a version 1.0 of the spec.

I certainly applaud the effort, but CommonMark did not unify all the Markdowns.

Cited in the spirit of good humor, but not irrelevant: https://xkcd.com/927/