Skill Challenges have been built with an eye towards the game mechanics and internally consistent logic. Challenge Ratings are normed against Skill Ratings, so that a specific Challenge Rating is a commensurate challenge for a specific Skill Rating. Also, Skill Ratings are built from both pluses (measured by how much training one has) and the Base Skill (determined by their Attribute.)
Here’s why things fit together the way they do:
The Base Rating for an Average attribute is 2. With Minimal training (or experience), +1, Average people have a Skill Rating of 3, Novices. An Average person with Minimal training is a Novice.
This is a common-sense, easily understood measurement. People with minimal training/experience are Novices. (Even the very talented but minimally trained are Novices: Base Rating 3 +1 = Skill Rating 4. Everyone, even those with potential, have to start somewhere.)
Average people (Base 2) with a Beginner’s training (+4) are Skilled (Skill Rating 6).
Average people (Base 2) with demonstrated Proficiency (+9) are Professionals (Skill Rating 11).
Average people (Base 2) with Expert training (+14) are Accomplished (Skill Rating 16).
Average people (Base 2) with a Mastery of the subject (+19) are World Class (Skill Rating 21).
Again, all of these are pretty straightforward and make sense. You can easily understand why a Master of a subject is World Class.
The rest of the Skill Ratings follow similar internal logic, as do the Challenge Ratings. Challenge Ratings are defined by how challenging they are, in relation to specific Skill Ratings. Difficult Challenges are apt for Professionals, for example. But the dice mechanics also come into play, specifically with how low you can roll.
A Professional (Skill 10) always succeeds at Routine (CR 0) Challenges, because the lowest you can roll is -9.
Accomplished characters have skills of 16+, and always succeed at Easy (CR 5) tasks.
World Class characters have skills of 21+, and always succeed at Difficult tasks (CR 10).
A Grand Master has a Skill Rating of 26+, and they always succeed at Formidable Challenges (CR 15).
Legendary characters (Robin Hood, at archery) have skills of 31+, and they always succeed at Grueling tasks (CR 21).
In-world, “always succeeds” means the task is effortless. The character is so used to doing this, that they don’t even have to think about it or exert any effort. The player can roll, to get a higher Success Rating, but they don’t have to. Even in difficult circumstances, they will succeed.
It takes a long time to climb the skill curve (by deliberate design choice). But once you do, there are great benefits. Automatically succeeding at low-CR Challenges is one of them.