It appears that the language detection feature in GitHub is off by a large margin.
My search for recently updated C# projects (https://api.github.com/search/repositories?q=language:C%23+sort:updated)
returned plenty of non-C# project labeled as C#.
E.g. https://github.com/haru-shinobu/StickFigure is labeled as C# 95.4%
ShaderLab 4.6% when it is just a bunch of Unity assets.
I estimate at least 10% of the results in the last few searches were completely off.