Most Building Officials come from the ICC world, that is their main training has been in the Building Code. In the Building Code "AHJ" does not exist.
The definition of Building Official is similar to AHJ
Building Official - The officer or other designated authority charged with the administration and enforcement of this code, or duly authorized representative.
Sarcasm- Most cities cannot be bothered with such trivial matters as establishing legal groundwork in enforcement.
Ask an inspector who the AHJ is, and I bet most will say him or her self, therefore you get wildcards making up there own Code. The Building Officials need to slap them down and take control, but that would actually mean work for a goverment employee. Sarcasm intended, but no I'm not bitter.
I would agree it is never the field inspector who is the AHJ except in tiny jurisdictions (if there are any) where the building department consists of one person.