It's how the duty finder system works. It always checks tanks first, it doesn't give a damn if there's 0 healers or dps in the que. As long as what it considers a small amount of tanks, like 3 or less maybe, then it's gonna show tank. I believe it checks the roles in the order they show up when you hit "commence," so it checks tanks, then healers, then dps. I mention this in the reddit thread you were talking about.