You don't actually get a random dungeon. If randomizes you to a dungeon based on the current dungeon demand.

For example. If we have 10 people in queue for dungeons, but all are queued for Sastasha. (think 10 DPS all queuing for this dungeon all looking for tanks or healers)
If you, as a tank in this example, try to queue it will see 5 groups queuing for sastasha and no other dungeons so it will put you into sastasha.

With that in mind remember that players in low level roulette are most often leveling side jobs or they are new players. So the earliest dungeons will pop up more often.