From the reports fine prints: "Estimates of the number of minimum wage workers in this report pertain only to workers who are paid hourly rates. Salaried workers and other workers who are not paid by the hour are excluded, even though some have earnings that, if converted to hourly rates, would be at or below the federal minimum wage. Consequently, the estimates presented in this report likely underestimate the actual number of workers with hourly earnings at or below the minimum wage."
Could it be that the structure of labor changed since 1979 with more salaried worker at minimum wage?
FWIW, you can't be classified as salaried and exempt from FLSA if you're getting paid minimum wage and working 40 hours/week or less. The federal limit for salaried exempt is equivalent to like $17/hour, and many states tie it to a multiple of minimum wage (e.g. 2x minimum wage in California)
If there was a significant % of salaried workers at minimum wage that would be quite a story. But this seems rather unlikely, unless you have some evidence to the contrary?
Could it be that the structure of labor changed since 1979 with more salaried worker at minimum wage?