i don't think all such necromancers would be satisfied by simply saying "it works sometimes and doesn't work others". maybe superstitions develop, or maybe a completely incorrect systematization becomes part of the occupational folklore. who knows. way more interesting that way
type O necromancer who's corpse explosion spell only works on the same blood type slowly learns they can get consistent results on enemies who their blood splattered onto, multi-classes berserker instead of learning the secret
the royal family holds up their immunity to necromantic attacks as proof of divine favor but it turns out to just be inbreeding
Magic system based on thermodynamics but most people learning magic think there's a precise hierarchy of demons you summon to cause magic effects and their system of true names and bindings works 67% of the time because the summoning ritual creates a pocket of equilibrium completely by accident