The real takeaway from this data is not just that the below average half tends to significantly overestimate its abilities (although that is supported), or that

The real takeaway from this data is not just that the below average half tends to significantly overestimate its abilities (although that is supported), or that the extremely effective tend to underestimate their abilities (also supported), but that, almost completely regardless of skill, people tend to estimate their own abilities to fall within the third quartile. To me, this seems like reasonable evidence for the otherwise anecdotal fact that (at least for some fields) those who are not themselves competent at a subject are poor judges of competency at that subject, since their primary tool for making skill assessments will be to assess the confidence others have in their skills.