You gotta think of it like skiing: everything out of bounds is a category above what you find in the resort, so within the resort, things are rated based on the relative difficulty. The stuff at Renz that is black diamond is fun, but it is more difficult than North Canyon road, which is probably more challenging than rancho road... so, Troll's shouldn't be a black if Windmill is a black, but windmill shouldn't be blue if North Canyon is a blue, etc etc...