When I first encountered AD&D 22 years ago, I couldn't wrap my head around how you make attacks rolls.
I still don't know why inverted armor class even exists.
It's because our favorite game derives from miniature wargaming of the time.
In alot of minis games, the more protected the mini, the easier it is to roll your armour save. Often on a d6. You're trying to roll that # or higher.
A save of 6+ is terrible. A save of 3+? Much better.
You can see this in action even today by watching a current game of 40k or Age of Sigmar (and before that Warhammer Fantasy Battle) from GW.
For example:
No armor = no save
Shield OR Light armor = 6+
Shield + Light Armor OR Heavy Armor = 5+
Heavy Armor + shield = 4+
add in barding if the models mounted on an armored steed for another +1
add in magic/cover/range/special abilities (like Dex) for additional downward mods depending on the exact game/edition....
Looking familiar yet??
In D&D the better your armor & mods, the lower your AC.
Now there's also mods based on the strength of the attack. These push the save back up. So if I hit your Save 3+ model with a weapon that has a +2 str bonus, now you need a 5+ to avoid damage.
In D&D this is where the str/dex mods to hit + that really awfully explained "weapons vs armor" chart come into play.