I emailed NYU and they responded saying that "We plan on informing all applicants of their decisions in March", at least I think this is true for foreign applicants. Maybe the mid May deadline might be for the waitlisted applicants.
As for the alphabetical theory, I don't think that is correct: 4 people from my office applied to NYU, the first one with surname starting with letter L was accepted March 2nd, the second one with surname S was accepted around March 6th and the third with surname with letter C was accepted on Monday. Last one also with letter C is still waiting a decision. I think the alphabetical theory is true for Columbia though.
Yes i think my theory doesn't work, so thank you for clarifying! I have no more theories then on how they do notify applicants!