Missing the obvious

If you try reading about Borderline Personality Disorder or Bipolar Disorder, or both, it won't take long before you encounter people explaining that people often confuse the two disorders. The explanations for why usually make sense. Some of the behaviors, for example, are superficially similar, especially when observed by people who are not mental health professionals or who are not well-versed in those disorders.

Those are good explanations, but rarely--actually, never, in my (very anecdotal) experience--do the explainers state what to me is one obvious reason for the confusion.

Both disorders begin with a "B" and both are commonly abbreviated as "BPD."