Roulette's first priority is completing groups for any dungeon somebody has queued specifically for, so if a dungeon is popular and gets specifically selected a lot, then people doing roulette will also end up in that one a lot.

If everyone in the group is there through Roulette, then a dungeon will be selected randomly, but it will be random from the set that everyone in the group has unlocked. Any group with at least one low-level character will of course have to get a low-level dungeon, even if the other 3 members are level 50. Add that chance to the fact that even when all members are high level, those same low-level dungeons are included in the random possibilities, and you end up with a higher "random" chance of getting low level dungeons than higher level ones, just because more characters will have access to the low-level ones.