since WOTLk came out no mobs have resists. they dont want it they dont do it anymore.
tbc sure there was plenty of mobs that could resist a type of magic, alar come into my mind about being fire inmune. huge robots in SW were nature inmune along with lootreaver (in the eye as well) along with bleeds effects.
this changed since wotlk, right now there is no mobs with high magic resistances to a school. simple as that, spell penetration by any means work in pve, not in your shots not in any other way. you can raid right now as any spec that you wont gimp your theorical damage by a resist mechanics.
in PVE there is no resist. traps are either miss or inmune. thats it.
they droped that model long time ago, right now in raids i never had any resist on my traps, trust me i done a lot of CC in raids and never hapen, including frost elementals (back them you would expect an elemental to be inmune to his own magic).
go outside, find a mob that give u a full resist on a trap post a SS and i will not post again about this.
in PVE we have another situation, isnt players fault, you would never get a resist in a player unless some buffs are present. mark of the wild, kings and resistance aura provide a large number of resistances, causing traps to resist, hence why you need 210 spell pen to avoid damage reduction by your target.
let say you are survival, your main shot is explosive shot, you hitting a target with his resistances in 0 you damage per explosive shot ticks is around 10k each tick, add in kings and that 10k will become a 9k, add resistance aura into the play and that 10k tick will become a 8k, add some spell pen (around 100) and that 8k hit will become a 9k hit and if u add more spell pen (210) that 9k hit will become a 10k hit. is pretty simple.
traps are not affected by this, we dont know if is spell hit that is causing the resist (i doubt it) but for sure is something only affected in PVP and not PVE.