@jfoutz - once your off employment insurance benefits, .gov doesnt 'count' you as 'unemployed'. You can look for work 24x7x365 and you are not officially 'unemployed', according to the gov.
I don't think it's connected to insurance benefits. I think the statistic is dumb, because it should count people who are available to work but aren't.
On the other hand, if you aren't looking, maybe you really aren't available.
If your collecting benefits your filing a report stating that you are/looking & available, if your not collecting EI benefits, your not filing report, your likely not counted as "officially unemployed".
You may fall into the broader U6 unemployment numbers that u/roofus mentions below.
Of course, I doubt both show underemployment.
Came across an interesting article[0] about how they are measuring these along with some historical data. It touches on underemployment as well, indicating that the underemployed are included in the U6 numbers.
The articles most recent(2015-06) monthly U3 rate of 5.3% is about half of the U6 rate of 10.5% for the same period.
[0] http://econbrowser.com/archives/2015/07/measuring-unemployme...