Mobs above your level gain resistances to all spell schools resulting in many partial resists. Those cannot be overcome by spell penetration.
Those resists are what mods like Glock showed, I guess MobInfo isn't much different.
It's been said in every other post in this and other threads :'(
On Supremus - that one really looks like some fire resistance on a boss.
He mitigated about 20% of all fire damage taken. That's around 97 fire resistance remaining after Curse of Elements (-88 resists) and talents (-10 resists) and whatever spell penetration they have from gear.
Given that ~25 of this resistance is level-based, you can grab around 70 spell penetration from gear for a ~20% damage increase.
Dalien just won the cake, but I sadly can't deliver to US servers, so this one will have to do
.iiii
(...)
