Here's what I decided to do for my OD&D game: Say that all giant types are AC5 naturally without any protection. Then say that the more sophisticated giant types customarily wear better armor, with the same number of steps protection that PCs get from them (or equivalently, normal book armor AC +4 steps). For example:
- Hill giants, AC5 base. Normally wear no armor (primitive skins) for AC5. If giants in the Steading get to the Armory for shield & helmet, they become AC3 (+1 each).
- Frost giants, AC5 base. Normally wear leather armor (berserker/barbarian-style) for AC3 (+2 steps improvement).
- Fire giants, AC5 base. Normally wear giant chain mail (dwarf/heavy foot style) for AC1 (+4 steps improvement).
Finally, if you compute the average AC among all these types in their standard armor, weighting by frequency of occurrence (6:1:1, per Vol-2, p. 9), then you get a mean value of AC 4, exactly as shown in the OD&D monster reference table for all giants.