Each point of res% increases your longevity more per point as your res gets higher. Let me illustrate.
For these examples...the attacker's base damage is 100, you have 1000 hp. The resistance is multiplied off of the base damage (10% damage resistance would take 10 off of the 100 damage, making it 90)
Resistance--Damage Taken per hit--Number of hits to die
0%-----------100-----------------------10
20%----------80-------------------------12.5
40%----------60-------------------------16
60%----------40-------------------------25
80%----------20-------------------------50
Notice how much your longevity (number of hits to die) increases exponentially per % of res? (this is also partially why the ability to tank was nerfed so much) Hitting someone with a tiny bit of -res IF -res subtracted straight off of the person's res bonus would be extremely overpowered, as it would dramatically weaken a mob/pc's chance with high res to tank when hit by it. That is why, say, a power that hurts def by 15% will take 15% off no matter how high their res is.
If it res DID work by subtracting the res straight off their res bonus, then someone who has 80% res...if they got hit by a -20% res debuff to bring them to a 60% res bonus, then they would take DOUBLE THE DAMAGE that they were taking before.
This is also why CoH has had so many balance problems before, and why a majority of other MMORPG's have their resistance formulas go by an "armor class" method rather than a straight % resistance to damage.
