So? That graph is about the capability of the system and how it improves with effort. This has nothing to do with which part of the system fails the most. Where do you see the contradiction?
Level 0-5 is terminology used by SAE to describe the range of fully manual (Level 0) to fully autonomous (Level 5). One would expect that in an article about Autonomous Vehicles, L{0,1,2,3,4,5} would describe the level of autonomy that the vehicle provides, but the article instead uses these to describe arbitrary thresholds of a graph demonstrating the capability of an AI.
I'll update at some point with different notation.
Not sure why l meant anything in some other model would affect its reading. Good to me.