Similar Situation, I think its happenign recently in Fairfax VA, one of my Dai also went for interview, He passed everything. He thought oath will be same day too. But he was given one white sheet of paper, that indicates, congratulation you are eligible for Citizenship bla blah.......he has to wait another letter at maill to get to know when and  where oath will be taken. That means another three weeks......Its so wiered.