I'm not sure, of course, but having worked in university settings for awhile, my guess is: 1) Someone screwed up and they just have forgotten to send rejections to a few of us because, I don't know, our last names are between j-n or something (that's the pessimist in me, and probably the person who's right) or 2) They have a few people they keep around until they have confirmation, so we're on a sort of "wait list," in case anyone for some reason turns down the offer, which might seem crazy, but life happens.
But really, who knows?