You're not missing something ... well, you're missing this feature
Currently, things qualify for a soup and go in in random order. That means you can't depend on the fact that the first 20 to go in would necessary be the highest sort order or lowest sort order, so it would be really tricky to only have the "bottom 20" or something like that, since we would have to add them all, sort them (which you also can't do in the releases you guys have), and then remove some of them. That gets hairy fast ... Because then if you delete a song, you have to trigger it to go look for the next lowest.
What might be nice to add into the search query is a keyword for "current time" so you could do something like "playlast > (curtime - 240000)" or something, so you can match all the songs you have played in the last x amount of time.
ms