It's not exactly a bug: When you train an administrator in the first place you really get 4 admin points and the admin is placed in the Administration slot of the Govern -> Economy screen. That gives you one more admin, so you get 5 admin points altogether. You can put a leader in the Adminstration slot which would give you also one admin point.
So when you promote an admin you get another 4 admin points, but since the admin himself is retired it is removed from the Administration slot so you lose that additional admin point.
But sure, the decriptions are misleading at the least ...