I guess Admission Office had already made the decision for most R1 applicants, as to the delay notification, I doubt whether they may have following concerns:
1. Some alumni could not provide interview evaluation promptly;
2. To avoid many applicants choose Duke as back up, so that they postpone to the date after most R2 deadlines.
A few days after you submit your application, someone, data specialist in the admissions office, will inform you whether your materials are complete or not.