the alternate list at udub seattle wwami program ranks people on the list, ie they pull people of the list in a predetermined order.
BUT, maybe some waiting list do that too?
what ever it is, the idea is pretty similar, "we don't want you now, but maybe we'll get a hold of you later".
best of luck.
btw, don't feel bad, most schools put a lot folks on these list just so they can get a better feel for the whole interview pool.