[Gregstrom]

And in This Thread, we get:

Originally Posted by PhilD

Back in Dom2, there was a "diminishing returns" protection formula, where protection values didn't exactly add up together. Putting, say, a 10 prot. armor on a creature with a 10 natural prot. didn't result in 20 prot, but something lower.

IIRC (but it's highly likely that I don't remember it 100% correctly), the formula was something like

FinalProt = 40 - (40-ProtA)*(40-ProtB)/40

That is, count a protection of 40 as "maximum", and, say, protection of 10 as "25% protection"; then protection values don't add, rather the "unprotected" percentages are multiplied.

Thus, a natural protection of 10 lets 75% go through, and an armor protection of 20 lets 50% go through - so adding the two together would let 37.5% go through, resulting in a protection of 62.5% of 40, that is, 25.

Now, I don't know how bonuses to protections would be taken into account with this formula, or even if it was completely correct, or if it could explain what you observed in Dom3...