I strongly suspect that AS is simply giving out outdated information when it says
Quote: If a target has 75% Cold Resistance and you use Skill Level 1 Cold Mastery, the target's resistance will be 75 - 75% * 0.20 = 60.Cold mastery's stats only make sense if it is doing a straight reduction of resists like conviction. I suspect also that cold mastery will lower resists below zero.