Wondering how that gets measured— the gig economy could skew this by a lot. In general it’s a tough road no matter the paycheck though.
Examples:
Age is a factor. How many "self employed" people are young up-and-comers who do not yet have the resume to actually make good money as contractors? What percentage of the "self employed" workforce is that? That's just one example.
Type of business is another. A one-person house cleaning business is likely to make a lot less money than a one-person niche SaaS company.
After thinking about it a bit, this stinks of "we have a conclusion, here's how we manipulated the data to support it."