I suspect that the "cap" is because these ratings are obviously based on how well cars do for their class. Every car has some sort of focus - this is an efficient car, this is a car with a lot of cargo space, this is a performance car, this is a safe car, etc. Sometimes a car will check two boxes, or do pretty good at three of them, or whatever. But when a car checks *all* the boxes, then it breaks the system. The system is not made for cars which get 2x the efficiency of a Prius and are also quicker than a 458 Italia. The system doesn't need to consider that possibility because it would be ridiculous to even think that a car like that were possible. That's why the system breaks, because we're dealing with a car that doesn't make sense. It checks boxes which ought to be mutually exclusive.