I also don't find it an odd question. Rather, I would explore his reasons.
It opens the patient up to sharing what they feel about their illness, and their diagnosis.
It doesn't mean they are saying they believe you are misdiagnosed. He was asking you for your thoughts.
I would start my next session telling them how it threw me for a loop, how I obsessed over that question, how I shared on psych central.
One of the things that happens to me (to us?) is we have so many emotions happening at once - especially when we get bowled over - that we need to slow everything down to filter through them, to get a handle on ourselves and regain stability.
I am very direct, and I believe in being totally honest with my doctors.
Lastly, my psychiatrist dealt with my meds, but my psychologist dealt with my emotions. She said once that she couldn't treat bipolar since it is treated with meds. But she had other things to focus on. I don't know if thats for insurance reasons - or if its just okay for psychologists to treat us too. I mean, so much happens in our life because of the bipolar, that we need someone to talk to about all the crazy shtuff.