Resistance Rune Inconsistent Math

When calculating resistances on my gear, I noticed that I had resistance runes calculating my total resistances differently. On my gloves with +24% cold resistance, my glacial rune was adding only 2% to my total cold resistances, instead of the 12% increase I was expecting.

When I equipped only my helmet with +31% fire resistance and a desert rune for +12% fire resistance, I saw a total calculated fire resistance of 43%.

One piece of gear is calculating the rune +12% resistance by percentage of the gears corresponding resistance, the other is adding it as a flat 12%.