I'm gonna take a stab at this. My post is going to be long so be warned.
The problem with unemployment number is that it is based on unemployed/labor force. Labor force is a very economic number because it has restrictions, such that discouraged workers (those who stopped looking for jobs) and those who haven't had a job in 18 month (maybe I'm wrong about the 18month) are no longer counted as part of the labor force.
Another issue is that retired people are not part of the labor force. In recessions, management offer early retirement to those near retirement age, and most probably have to take it even if they want to work.
Since actual unemployment rate (U/L) is not 1, the factors above reduce the actual unemployment rate because it decreases U and L 1:1
Furthermore, underemployed workers (part time workers) are counted as employed. In any economic downturn, people would gladly take part time job as opposed to no job, so again the unemployment rate is skewed downward.
All in all, the unemployment rate we see is severely undercounting the actual unemployment rate and the number of unemployed Americans.
Official numbers might say 15 million, I wouldn't be surprised if the actual number is over 20M if we count those who work part time jobs but want full time jobs as unemployed or count every 2 part timers as 1 unemployed.