That's because when you play against the AI you probably usually have at least 3 of the 5 global slots.
But it's happened too often to be chance. If all 5 slots are full I have *NEVER* knocked out a random AI spell if one of my own is up. At this point I no longer even try.
Quote:
I frequently see the AIs fight over global slots, sometimes for the same spell, sometimes a rivalry between Eyes of God/Dark Skies/Maelstrom/etc.
-Max
Yeah, the AI's do take out each other's spells at times.