It may be mod-specific, but I'd still like to know. Plenty of times, I've taken dozens of hits from an Orespawn or Godzilla monster and been fine, and suddenly on the next hit I'm dead. Since I play with keepInventory, I know the armor hasn't broken. What's happening there?
You died because you took to much damage. Armor provides Defense Points that reduce damage taken. Armor loses Durability from any damage source that can be blocked by the armor. If your taking more damage than your armor can absorb, then it reduces your Health. Plus, Armor does not provide protection from all damage sources.
As far as I know, including from looking at the vanilla code, there is no connection between armor durability and how much protection it provides, even if it has 0 durability left and breaks, just as weapons deal the same amount of damage even on the last use.
Also, when you see mod armor that provides more than 20 armor points from a full set it is basically wasting armor to wear a full set since vanilla limits the number of armor points to 20 - even zombies, which have 2 base armor points, are still limited to 20 (not 22) in full diamond armor, and a single piece that provides 20 points by itself should be the same as full armor (the game also doesn't care if part of your body isn't protected; e.g. getting hit in the head without a helmet on does not deal more damage than getting hit in a protected area).
Of course, this assumes that mods don't change armor mechanics; for example, I changed armor durability loss so it only loses 1 durability per hit regardless of how much damage is dealt (vanilla takes damage / 4 points away; e.g. 100 damage takes away 25 points; more usually mods just give armor some insane amount of durability so it lasts a decent amount of time when fighting bosses with extreme attack damage).
The video posted by PlantDoesMC seems to suggest that the armor is blocking all damage since with vanilla limits you'd still get 20 points of damage from a 100 damage source (80% damage reduction), thus be unable to survive such high damage without enchantments and even then you'd still lose a good deal of health (enchantments also normally stack to a limit, so making armor with every possible enchantment and/or at levels higher than achievable in vanilla won't be better; for example, Feather Falling V is enough to reach the damage reduction cap for fall damage. Mods may also modify this).
You'll have to check the mod's documentation or ask the creator if armor works any differently to be sure; some mobs may also have attacks that penetrate armor (e.g. Guardians in vanilla 1.8).