Spirit Plague Damage Scaling Bug?

My Spirit Plague Lich character has 86% increased spell damage, 40% increased lightning damage, and 100% increased necrotic damage.

But for some reason, a Coral wand (+51 Spell Lightning Damage, +45% chance to shock on hit) is giving Spirit Plague more damage/DPS (tooltip) than a Profane Wand (+63 Spell Necrotic Damage, 57% increased curse damage).

Is this a bug?