My anxiety diagnosis pre-dates my BP one, but my BP one was a long time in coming, so they may well coincide. One has to keep in mind, that symptoms don't always coincide perfectly with diagnosis: you may get diagnosed later, whether months or years.

I agree with you that some diagnoses that are diagnosed separately are really part and parcel of one (in this case BP and GAD, but I think it can happen with other diagnoses where there is a primary one and then others). As you say, it just makes sense. Though I don't think it's the case for everyone.