It's not just a matter of prioritising the people, they also have rules about what they can offer and they have rent caps for families and single people too, so a 4 bedroomed house can't be offered to a couple with no children, because it's not suitable accommodation.
If you turn down an offer of accommodation you go to the back of the list. I wouldn't be surprised if that house was offered elsewhere and refused because of the rent top up that would have to be paid by the tenant for a house that's too big for them.