I believe the reason is that the *.github.io hosts point to a CDN rather than just having a single A record, and it is only when going through the CDN that you

I believe the reason is that the *.github.io hosts point to a CDN rather than just having a single A record, and it is only when going through the CDN that you bypass the "neutering". Regarding your second question, it seems that github issues a warning if you do that: https://news.ycombinator.com/item?id=7738913 https://news.ycombinator.com/item?id=7738913