A set of specifications requires stiffer forms for the exterior surface of an architectural concrete wall than the interior. Is this common, and if so, why?
You did not say whether the interior surface will be exposed to view, which could make a difference. Forms for architectural concrete should be stiff enough to minimize deflection and make any variations from plane less apparent. Deflection of forms rather than flexural stress or horizontal shear may be what governs form design in this instance.