Interviewing "to augment a mostly-made decision" seems sensible. I would expect they choose to interview those people who look like good candidates on paper, simply to ensure that they sound relatable and answer some questions they don't ask in the application. You pick up tone and ease speaking with others (various things) that you can't when speaking vs just in print. I can't imagine they just interview everyone. (I do know they only interview PMHNP and CNM).
LCSWtoPMHNP, could something have happened during your interview to throw things off?
It's possible you simply had someone who felt the need to make sure you nothing was set in stone...that the interview didn't mean anything for certain.