Lawrence Mishel, an economist for the left-leaning Economic Policy Institute and a long-time skeptic of the significance of the gig economy, said it wasn’t surprising that the report showed relatively little impact from online marketplaces.

“A lot of this hype has been driven by the tech world believing that they are the center of the universe,” Mr. Mishel said.

But Mr. Mishel said he was surprised by the slower growth in other areas. And he wasn’t the only one perplexed by the report.

The Bureau of Labor Statistics, for budgetary reasons, had not conducted its survey of alternative work since 2005. But in 2015, the economists Lawrence F. Katz and Alan B. Krueger conducted their own survey that tried to mirror the bureau’s methodology. They found that alternative work was more common — and growing far more quickly — than the data released on Thursday suggests.

Economists offered various explanations for the discrepancy. One possibility is that the boom in gig-type jobs was real but that as the economy has improved, more people have been able to find traditional work. Part-time work, which surged in the recession, has fallen in the recovery, and employment by temporary-help services has leveled off. If true of alternative work more broadly, that would suggest that what many commenters interpreted as a structural shift in the economy was instead a temporary result of a weak labor market.

It is also possible that the new data understates real changes in the nature of work. The government’s standard tools for measuring employment have struggled to capture the shifting employment landscape. For example, the Current Population Survey, the monthly survey used to calculate the unemployment rate and other key measures, shows that self-employment is falling, even as tax data from the Internal Revenue Service has shown the opposite.

“The questions on our standard surveys don’t probe into the nature of these arrangements,” said Katharine G. Abraham, a University of Maryland economist who served as commissioner of the Bureau of Labor Statistics under President Bill Clinton. “We’re not asking the right questions, and they’re hard to answer anyway.”