geography != politics
> seems to preclude NI from being part of GB but both are encompassed by UK.
Technically GB is a geographical term, referring to "the big island off the coast of Brittany". We can probably blame the Normans and their descendants. They had Bretagne (what English-speakers now call Brittany in what we now call Northern France), and having taken over a goodly chunk of the island next door they referred to it as Grande Bretagne (Big Brittany), which could perhaps have been better translated as "Greater Britain", in the same way we talk about Greater London, rather than Great Britain. Ireland came along into the political fray a bit later, and the whole idea of the UK much later again.
By the time ISO got involved they should certainly have used UK, not GB, but then the French think that "Angleterre" means the whole island anway, and most are barely aware of the existence of Welsh or Scots, let alone the complexities of we Norn Irish.
C'est la vie.