Hi, Billy Mays here! Questions like these usually should be in the Character Building Helpdesk, which is in the Character Building Group. To answer your question, this is because you have some sort of Spell Absorption effect. This can range from the Atronach Stone to Atronach perk and Miraak's clothes; it could even be a combination of all of them.
Spell Absorption affects all Summoning Creature spells and staves that summon daedra/dwemer automaton (if you have the Aetherial Staff).