This is basically because the increase in non-regular employees reflecting the increased labor force participation of women and seniors is working as labor slack. In other words, it is causing a slackening of the labor market even amidst an intensifying labor shortage. In fact, the number of employed persons continues to increase while being accompanied by a rise in the labor force participation rate.
Nelson, Benjamin, and Misa Tanaka, “Dealing with a Banking Crisis: What Lessons Can Be Learned from Japan’s Experience?” Bank of England, Bank of England Quarterly Bulletin, 54 (1), 2014, pp. 36–48. Nishimura, Kiyohiko G., “Macro-Prudential Policy from an Asian Perspective,” remarks at the High-Level Seminar on Macro-Prudential Policies co-sponsored by the People’s Bank of China and the International Monetary Fund, Bank of Japan, 2010.