In a nutshell, Hexen's armor system is just that. SavePercent is dependant on SaveAmount. If you have 100 armor, armor reduces 100% damage. If you have 50 armor, damage is reduced by 50%. 25 armor means 25% damage absorption, and so on.
Tbh, I wish all other Doom games' armor was like that too. The better the condition of your armor, the more it protects you. So that at first Green Armor would be as effective as Blue Armor.
Of the top of my head, the only other two games I recall which have a similar armor system are Elder Scrolls Oblivion and the S.T.A.L.K.E.R. games.
The status bar or fullscreen HUD's armor display only adds insult to injury. It only displays 1/5th of the actual armor, rounded down. Use the alternative HUD to see the true value. It's not that hard to understand, once you familiarize yourself with it.
https://zdoom.org/wiki/Classes:HexenArmor