Jim,
The system is a blend of course rating and slope rating. Slope rating shows how much more difficult a course is for a bogey golfer than for a scratch golfer. It stands to reason that that variation in difficulty shouldn't cease at a 0 handicap. Take Bethpage Black for example. From the blue tees, it is rated 78.1 and 152. A scratch player would shoot 78 to play to his handicap and an 18 would shoot 102 to play to his. A +6 would need to shoot 70 to play to his handicap. Bethpage has quite a lot of trouble around and it's long (7,462 yards from those tees). So it's difficult for a scratch player and very difficult for a bogey golfer.
Take two courses, both are 7,000 yards. One has trouble all over the place, water, deep rough, trees, you name it. It, like BPB is going to be hard for the scratch and hard for the bogey golfer. The other is wide open and without any real trouble. That's going to be easier for the scratch player and much easier for the bogey golfer. Those two courses, the first might be 76/150 and the second might be 73/125. Harder for a scratch on the first, but marginally so. Much harder for the bogey golfer. The hypothetical +6 is most likely going to find both of those courses equally difficult. Unlikely to be troubled too much by the added trouble at the first, but you still have to hit the shots and hole the putts. So how does that get factored in? By extending slope downwards as well as upwards from zero. So the three players, bogey, scratch and +6 would see scores to play to handicap of 100 and 93 for the bogey, 76 and 73 for the scratch and 68 and 66 for the +6. So the +6 would beat the scratch by more on the higher slope course. A mythical "perfect" golfer would shoot the same score on both courses and would therefore beat everyone by more on the higher slope course.