Bipolar Disorder Support Group
Bipolar disorder is not just a single disorder, but a category of mood disorders marked by periods of abnormally high energy and euphoria, often accompanied by bouts of clinical depression. This is the place to talk about your experience with bipolar disorder, learn from others' experiences, and find support.

I think the answer is that doctors don't really recognise that difference - if you are depressed and are depressed for reasons other than a situation then it is part of bipolar.
People don't get for example a diagnosis of bipolar AND unipolar depression.
It's an odd situation but we need to remember that because there is no real biology/ causes identified doctors just have to work with symptoms.