Since the symptoms of each are very similar, i.e, cycling, depression, hypomania, they are treated with the same meds. Bipolar has genetic roots so it's primarily treated with meds followed by therapy where BPD is treated by therapy first and meds are used to control the symptoms.

Bpd cycles or mood changes are more or less "triggered" whereas bipolar runs on automatic. Also, ultra rapid mood cycles are more closely associated with bpd. There are exceptions though like me. The only thing that gets triggered in me is anger, but I'd have to be in a mixed mood to get triggered like that.

Since both presents about the same, misdiagnosis between the two are quite common. A person can be DX both though, a dual diagnosis is not uncommon for those with bipolar.