It isn't, and as far as I'm concerned the residential density is fucking borked, thus the underemployment. I think it'll be changed. As it is I do not build industry at all because I can't even fill commerce jobs and industry is gay. Just build your state-owned shit like mines/wells/processing.
Of course there is this:
Take a look at all the data and there is defiantly something going on here...
I have 10,982 workers 5174 shoppers and 3124 in school.
I have 11,000 unfilled jobs and 21,000 unfilled freight orders.
However...
Not a single building has shut down to lack of workers, all my services are running at max capacity, my schools are filling up and commercial businesses are still building.
So even with the lack of workers and massive lack of freight the only thing my city actually needs more of is shoppers.
Commercial is building then closing down to lack of shoppers, not workers or freight.
I have a feeling its a bug...
Another thing i noticed between workers shoppers and school i have ~19000 people. I have 109k residents, so i pull up the population map that shows me how many workers/shoppers/residents/education/etc. and I see that even a rough guess on what the map shows me is... ~5-10k people are 'at home'
so ~19000 people doing things and another 5-10k at home does NOT make 109,000 people.
That being said there are a lot of residents moving from place to place... but i've already counted those as those are workers going to work and are calculated in the 'workers' stat on the jobs break down. Same with shoppers and tourists... leaving 70,000 people missing....
The phantom denizens of your city that exist nowhere but on paper.