I think the way "roulette" works is that it takes the least popular duty (the one that the least amount of people are queueing SPECIFICALLY for) and puts you in that.

However, I do agree, if it does work like I suspect, I think there should be an equal chance that you get any of the duties listed. However, I think along side that, I think they should remove the adventurer in need bonus from the roulette, and make "Adventurer in Need" it's own queue with it's own reward. You don't know what duty you're going to get, but it is the one the least amount of people (of your specific role) are queueing for. Perhaps you could exclude DPS from this queue, though. They're USUALLY not needed.