When queuing an incomplete party, you're first filling spots for someone who queued for a specific dungeon that nobody is queuing for. As Fish points out, more people have access to and are queuing for lower level dungeons, which increases the probability that you'll end up in an early dungeon. Then, there will probably be more people queuing for Stormblood dungeons than for Heavensward dungeons, due in part to Stormblood being the current expansion, and because people can choose to skip to Lv60 if they want.


If you queue a complete party or end up matched with all roulette runners, random is random.