The reason is very simple, really, and tied to what the BLS survey asks of the individual respondent. They ask about the nature of the respondent’s sole or “main” job—the job they spend the most hours per week doing. While the BLS regularly tracks how many people hold multiple jobs, in the contingent and alternative work arrangements survey, they did not ask about whether any of the job holder’s additional work qualified as contingent or alternative.
Here is where we realized that tax return data can tell us something about the labor market that even the official employment data cannot. In principle, all taxable income should be reported on one’s tax return. Tabulations provided by the Internal Revenue Service’s Statistics of Income (SOI) revealed close alignment between the numbers from BLS’s contingent and alternative arrangements survey and the numbers of persons reporting self-employment income as their only source of earned income on their tax return, as shown below: