The scale they use may be based on the vehicles and the trails they run in the Jeep Jamboree events. It is probably the hardest trail trip they offer and maybe pretty close to the hardest trail doable in a stock vehicle. I would love to run it some day to see for myself.
It is based on stock Jeep. Although even then it's a bit overblown as Jeep Jamboree has had lesser Jeeps then the Rubicon in their fleet. I still remember when they had YJ's. All they did was add 31's and some skidplating. According to their own scale, the Rubicon can't do the trail as it woukld be limited to 6-7 unless you're saying the 1" factory lift is a 'lift'.
http://jeepjamboreeusa.com/trail-ratings/10
Extremely demanding. Reserved for the toughest off-road trip in the U.S.A. — the Rubicon Trail. Requires 4WD with 4-LO. (Not recommended for Jeep Patriot, Compass, Liberty, Cherokee, Grand Cherokee, or Commander.)
If you ever do the Rubicon Trail, you'll see that most of the trail is pretty tame. There have been done in even a stock YJ before for a Jeep or Jeepers Jamboree. (There are two big events at the Rubicon, Jeeper Jamboree and Jeep Jamboree which are usually a week apart) Rick Pewe of Peterson's 4Wheeler drove the trail in a stock GPW several years ago.