It's a long time since I was a cadet, but my impression at that time was that making a verbal greeting to a superior officer, unless they'd made one first, would be seen as a breach of discipine. Saluting an officer, on the other hand, was mandatory. (I'm not sure about returning a salute, but it seemed normal practice.)

So with the non-verbal greeting, the junior person initiated it, but with the verbal communication, the superior person would do so.