I think the issue is that the vast majority of people apply for a single room, meaning although you can apply for a twin room I doubt many people actually do so meaning there will be spaces available. There are only X amount of places in the university meaning some people who wanted a single room will have to be placed in a twin room so there is room for everyone to live there. I think the number of twin rooms varies each year - I certainly never met anyone who shared a room in my first year but I know people in the year above me, and the current first years who shared rooms so it probably depends on intake. It's completely the luck of the draw where you end up. It's not done on a "this accommodation is full so we'll move onto your next choice basis" as that's not fair on people lower down the pile, and also the conditional offer holders who won't be allocated a room until August.