Is the data that only 4-6 percent of American workers work a second job really accurate?

This data seems to consistently counteract what I’ve seen reported here on Reddit as is it possible that the data collection sample size is too small or skewed or the question itself doesn’t include workers who consider themselves working a “gig” even though it might be a actual second job? Overall, I’m just very baffled by this statistic and looking for some clarity here on the actual number