Three or more slashes automatically engages a browsers' file (local disk) protocol.

Typing ////users/me/desktop automatically takes me to
file:///users/me/desktop which is the directory for my desktop on my local machine.

I don't know or understand how it's sending you to the correct HTTP address.
Perhaps you're using something like Firefox's URL fixer or something of that like.

Regardless, someone made a mistake when they were coding their site on their local machine.