Ok, this is one of the most interesting posts I have seen in a LONG time. I know our accounts are at core, a base number that we never see. So the Btag change does not change the base.
If the base number changes that…that would be illogical.
The Btag part should be reusable, but the core account ID should be persistent.
As for the Btag part. There can be as may "Bob"s as people want.
So while the text shows “Bob” the number differentiates them. You are right though, those “should” not be re-useable or it screws up leaderboards, armories, etc.
The starting number also tends to indicate Region, but is not always accurate.
Again, the number in the Btag is NOT the core account number that Blizz uses.