I suspect that's because the nature of "not-unemployed" is changing. Long ago it usually meant either being out of the workforce (retired, student, homemaker) or a full-time job with benefits.

Nowadays we're seeing a decrease in the last category, and an increase in "underemployment" and people holding multiple low-quality jobs to make ends meet. They aren't "unemployed", but they're not in a strong negotiating position either.