Game Mechanics: Pegasus Feather Cloak using mana?

[Talori]

I don't know if it has been doing this all along, never took notice until today. I am full mana, clicked cloak which I keep in my inventory, and when spell is cast I go down 13 mana. I have repeated this several times now to make sure, and if I am full mana, it goes down by 13. If not full mana, it keeps going down to the same number. Example: I have 2489 mana, click cloak, goes down to 2476, click cloak again, goes to 2476 again.

Just strikes me as odd because it shouldn't use any mana at all....