I think one is a portal for GitHub developers, while the other is a public polished site. I reminisced the early Google forthright attitude that made life so simple and human.

The readme at https://github.com/google/building-secure-and-reliable-syste... does say this book is found there, and sure enough it's the first one on the list: https://google.github.io/building-secure-and-reliable-system...

It seems this github.io URL is more like a CI run of the book, and the one on sre.google is the "published" one.