Back when we were messing around with it in release season, I couldnt see a big difference in resists on gear. Having Poison resists on your gear didnt seem to really decrease how fast a poison pool would kick your ass. The next one was Paragon, and if you strip off all your Paragon resistances - poison puddle would almost one tap you at any reasonable NM level, with or without gear. Strong Elixir of Poison Resistance is 25% iirc, and without paragon resistances, and that on - it was a subtle improvement but not much.
Point is, its all fucked up. They may be calculated on the paragon pages correctly, but only additive 2.5% for a 25% elixir and probably the armor. Hell, I personally thought the armor shit wasnt being added to the calculation at all. If so, it was an additive addition of 0.25% for 25% resistance. Or, in some really fucked up dumb ass way - they were using additive values - but the max ceiling for resistances was 1000% instead of 100% with calculations.