Crystalion I have a "fit" for your data. It looks like the game first adjusts the damage according to the target's resists:
damage = damage*(100-resist%)/100
then if target's resist lightning < 0 make a second adjustment to cancel the first adjustment:
if resist% < 0 then damage = damage*100/(100-resist%)
however the game counts the -50% enemy resist from the wand in the first adjustment but not the second one (or the if <0 thing).
This seems to explain your results. Note I have not checked anything in the game, I am merely making up an explanation to account for your data.
damage = damage*(100-resist%)/100
then if target's resist lightning < 0 make a second adjustment to cancel the first adjustment:
if resist% < 0 then damage = damage*100/(100-resist%)
however the game counts the -50% enemy resist from the wand in the first adjustment but not the second one (or the if <0 thing).
This seems to explain your results. Note I have not checked anything in the game, I am merely making up an explanation to account for your data.