Well, they know you could do it, and they give out more offers than places available.
I did not decline any offers yet, I want to keep my options opened. Only one expired because I did not pay the deposit (Bristol). I am still somewhat unsure, esp. between Oxford and LSE, both have pros and cons, and Edinburgh is still a good back up. Who knows what may happen, they keep people "waitlisted" why can't we keep them waitlisted, too?