Leafing through the AD&D Monster Manual, Monster Manual II, and the Basic/Expert D&D rulebooks, it strikes me that the assignment of Armor Classes is disconcertingly arbitrary. Why, for example, would an orc have an Armor Class of 6? They have neither natural armor nor superhuman reflexes, so what accounts for this? If the Armor Class represents armor worn, why don't the descriptions mention it (as they do for elves and dwarves in the Monster Manual)? If damage can be described as "by weapon type," would it not be logical to describe Armor Class as "by armor type" for those creatures who wear it and have no other advantages?
That would solve the problem if it were the lone problem with Armor Class assignment. Why does a gelatinous cube have an Armor Class of 8? It's a 10' cube of protoplasm that causes no damage when it makes physical contact. It should have an AC of 9 or 10 (depending on the edition), i.e. equivalent to an unarmored person. If anything, it ought to be easier to hit, perhaps even impossible to miss.
Why do ogres, who are "human-like creatures" who "wear animal skins for clothes" (Basic D&D), have an Armor Class of 5? If their skin is as tough as chain mail, it that not worth mentioning? The Monster Manual says "They care for their arms and armor reasonably well." Does that mean ogres wear chain mail? Where do ogres get suits of chain mail that fit them? Surely creatures of "low" intelligence do not make chain mail armor.
In the future, I will be treating the Armor Class of all humans, demi-humans, humanoids, and the like as "by armor type," but I think I'll need to reevaluate the Armor Class of most other monsters to see if the rating is justified by their description or abilities. If not, I'll decide on a case-by-case basis whether to change the Armor Class or alter the description (or both).
The side benefit is that it adds an element of uncertainty where player knowledge is concerned.