New Necro pets vs Ultrablast

[Kozzloz]

I'd first like to say it's great that necro's have more of a variety with pets, and they are in a more accurate order.

I've noticed, however, that most of the undead npc pets are resistant only to ultrablast. Why are they just set to ultrablast and not other skills like deathfield, mindblast, pforce etc?