There are always a small number of individuals who submit a deposit for the Fall and then decide after May 1 not to atttend UD for a variety of reasons (I would gues most commonly deciding at the last minute to go elsewhere even though they would lose the deposit). Perhaps UD has been waiting to see if they receive notification from any of these individuals. If so dorm space would become available and it would make sense to move some of the Spring admits to these spaces (they would need to have the space for them in the Spring anyway and why not get the tuitition/fees for an additional semester). Of course this is just a guess on my part. Unfortunately I have not been very lucky with my guesses this year with all the craziness we have seen in the admission process. Good luck.