If you want a wild guess, some units (such as fire units) could have negative cold resistance, thus making them more vulnerable to cold damage.Maybe this question has been asked before already, but I've a curiosity on cold damage - how much different is it going to be from "phsyical Damage" of the same amount?
It strikes me that Cold - Rarly Encountered, Rarly Resisted, is almost identical to pure strength which is Always encountered, Never resisted.