I think it is definitely playlist related. Here is a quote from one of the developers:
The biggest thing that I can think of off the top of my head: how we set up our [matchmaking setup] hoppers. We changed that quite a bit based on feedback. We'll eventually have cleaner, more pure hoppers, by and large. There will still be one or two that are more mixed. But the feedback on the hoppers in the beta was really quite clear -- it was, "Why am I still playing Team Slayer in an objective-game hopper?" Hey, good point -- why are you playing Team Slayer so much in an objective-game hopper? So we fixed that, and we're paying close attention to that.
I guess it is the way that Bungie refers to playlists, but I haven't heard any other developers refer to them that way.