This is because during a recession the no. of marginally attached workers increases, as people try to do part-time jobs etc.
However, at the same time these workers are not included in the official unemployment rate.
Interestingly, the unemployment figures are exceptionally important, both for policy makers as well as politicians. They are seen as a snapshot of the health of an economy, the number of businesses, and overall growth.
Policy makers study the figures to plan out the next five to ten years.