Were you eating in a lot of sushi places?

Miso shiru varies from region to region, and, supposedly from family to family (the ideal is that a child can pick out his/her mother's from the multitude). I've made dashi for miso shiru using shaved bonito plenty of times, I still haven't tried making it with dried sardines.

Also the additions to this soup will commonly vary according to the season. Lots of tiny little slimy mushrooms make it into the soup in October, and little clams are common in the winter.

I suppose that restaurants that serve sushi and sashimi end up with large quantities of good quality fish pieces near the bone and these will make a great addition to a stock.

Just my own conclusions from observation.