An inanimate, immobile object has an AC of 10 + its Dexterity modifier (-5 for no Dexterity) + its size modifier.
Is there any event that would cause an inanimate, immoblie object to even have that wouldn't convert it into an animate object instead? Or were the writers just saying, "everything starts off at 10 AC, but inanimate objects really just start out at 5"?
(My usual Disclaimer applies here. I'm not a rules lawyer. Although I'm starting to enjoy picking apart the system this way)