expel undead getting resisted

[Adron]

I'm new to cleric. I saw that expel undead had a -50 magic modifier and assumed that when i went after Spectres (37 max) at lev 40, that I would see little or no resist.
Instead i've been completely resisted 3 out of 40, and partial resists on 6 of the remaining 40. 9 of 40?
I have of course been using root and spook the dead to not get gibbleted, and have never had eiither of them (20 casts total) get resisted.

Is it a waste for me to keep better track ,or is this just the way it is?