Its as simple as this:
-Draining 1EU per tick unless its redstone powered.
-Every 32Ticks (1,6 seconds) it will look at the total storage & devides it by 400
-This number is the damage if it is less the 1 It will skip it.
-If it is bigger then it will check in a 9x9x9 radius.
-Every entity it finds in this radius that is not wearing a Hazmat suit gets the damage it calculated before.
(So 10000 / 400 = 25 Damage (12,5 hearts)) that will reduce since you need to fill it up faster.
And it does not matter if you have 1 Entity or 5Million entities in that range. Every entity gets the damage applied. Also the same amount.
The thing you are discribing shows only that the zombies that survive the first shock hat some kind of protection. So they gained some shields from the armor & did not get so much damage as others