Armour class is the headline number, but it is not the whole story.
Class is a threshold, durability is the buffer
A class 5 plate at 20% durability does not behave like a class 5 plate. Degradation is why two players in nominally identical armour can have completely different outcomes against the same round.
What we can and cannot tell you
Arena Breakout: Infinite does not publish its penetration formula. Where this site holds a recorded penetration figure for a round against an armour class, it shows that figure and says so. Everything else on the armour calculator is an explicitly-labelled model, shown with its assumptions, as a comparison aid only.