It's not really random at all. The roulette checks which dungeon happens to need the specific classes the most and pair you with them. That's why you'll see so much Toto-rak, Tamtam, Qarn and Cutter's. I've also noticed the dungeons you get the most depend on your role.

Roulette was made to help people who queue without a full party find people to play with, even if it's old content. It's working as intended.