your case is similar as mine. The professor just wants to interview another applicants and will wait for my decision in middle of March around. Since the pact of 4.15 is the deadline for university to receive the decision of acceptance for the offer, the professor leave one month in advance before the deadline to wait for my decision.
But I have the same question if professor really want to recruit you as PHD student, he should send the offer anyway no matter if you accept it or not. He could just wait until deadline on Apr.15.