It just depends which are the most popular dungeons since these can be queued for on their own too. Ex Roulette will put you into whatever has slots available in the queue so it'll be the ones least popular a lot of the time. Honestly I've had a mix of runs often Qarn though more recently seeing way more Snowcloak or Sastasha. I think Snowcloak is the most popular to run since it can be done pretty fast so you'll probably see it the least on roulette because of demand.