Something to take in consideration, and you'll see a billion posts here about it, is that being "hit" and losing "HP" doesn't actually mean taking a wound. HP are an abstract concept used for combat that is hard for many to wrap their heads around, because it wasn't ever meant to represent a creature's physical body. Mechanically it serves as the balance to the increase in the frequency of being hit, rather than AC, meaning that everyone hits more often (which is fun) and doesn't die as fast (which is also fun).
My suggestion is to look at HP as a character's stamina (or luck/skill/plot armor/whatever) to withstand some abuse that isn't necessary a direct wound (such as stress, glancing blows, armor taking the impact, etc.). Of course as a DM, I usually describe most enemies loss of HP as actual wounds, since after the combat no one will care about their physical condition, unlike the PCs.
First of all, remember the goblins would have 2 less AC if they're using their bows, since they can't use their shield, and that dropping/reading it takes an action. This means that ones that start as archers, but get trapped in melee will have less AC than those charging into the party.
Secondly... something might be wrong with your characters AC computation, because this seems odd. Unless you're using the array option with character starting packages, most characters will start with AC: 14-16, except for sorcerers, warlock, and wizard. The array lends itself to lower AC, since you will probably have only a +1 in Dex unless you're a bard, monk, rogue, or dex based fighter (such as an archer). The starting packages lower AC because the armor given isn't usually the optimal choice (barbarian gets none, for example).
