It does sound unusual to have unipolar depression diagnosed together with bipolar depression. At the same time, considering the fact that your diagnosis has changed each time, maybe, my gut impression that is, is that maybe there's uncertainty about giving you the bipolar diagnosis, even though it lingers, from I to II. Dunno. Hopefully someone with experience can help better answer.
Can you ask your dr at your follow up appointment or address it with your therapist?