Keep in mind that Navajos and 402s are working airplanes. Cape Air's planes fly all day, every day.
Most 340s/421s are privately owned and don't fly as much. Also, charter outfits aren't usually flying every day. I know we didn't. 2-3x a week utilization for a plane was pretty good. However, the planes are still seeing 100-300 hours/year from many of their private owners who use them for both business and personal travel. Of course, even at that number, they're probably flying once a week at most.
Then you see the low time ones that have been sitting most of their lives, and that exists, too. You also have some people who bomb around in them VFR squawking 1200.