Why code download from website is different to github?

The code download from Downloads – Cesium is different to GitHub - CesiumGS/cesium at 1.89

And I notice DataSourceCollection.js import when from "../ThirdParty/when.js", but when.js does not exist in github. And I can find when.js in the code download from website.

I’m very confused about this. Thanks to everyone who can help me.