I noticed that recently as well. Was wondering what it was in the logs on the site, then I noticed it when looking through the things after running a test.
The referer header is the page the browser was on before navigating to the page. All of the testing starts with the orange.html page and then navigates to the page being tested.
The navigation used to use Chrome dev tools to drive the navigation which probably didn’t populate the referer at all. A while back it was changed to use window.location so it is consistent across browsers and it appears the browser passes the referrer in that case.