Arrogance (and Dunning-Kruger) comes with many secondary maladies like inability to recognize their lack of ability, lack of self-awareness, cannot objectively evaluate their (or others) competence or incompetence. This is exemplified by the political left, teenagers, or progressive internet trolls, who have motivation to "Fix" things long before fully understanding the problems, or the consequences of solutions: change first, think later. (Of course the right is not immune, but being that they trend older and more jaded towards unintended consequences, they do score better on this scale on at least understanding that they don't know what they don't know).
The graphs are oversimplified visualizations (constructs to understand the issue), it is of course not literally true. There's a scatter plot with those more assured and less, and more competent and less, and you'd find that many teens, progressives and leftists who know the least are the most assured not based on technical competence but based on consensus of beliefs and feelings. And in in the top-middle of assuredness, if a large pool of technically competent. The one thing in common is that many more people think they are above average than actually are (the Mount Stupid effect).
There is another phenomenon that overlays this one. Experts often grow to be highly assured in not only their field but in other field. So people get cocky because they are experts in one field, but apply it to others. More than that, their knowledge sometimes freezes. They did know a lot for the time, but they don't keep up with advancements, new learnings, or they develop ego-drive blind spots to new revelations. (Einstein couldn't get to his grand unified theory because he refused to integrate probability and quantum mechanics).
Thus both the competent and incompetent over overvalue their assuredness, the difference is the former are better at arguing their points and have more domain knowledge to back them up. But that doesn't make them less wrong. Especially in areas where they're transferring their domain knowledge in one area, to making them think they're smarter than everyone else, in everything (or anything) else.