Thematically it may be confusing, but from a balance standpoint, you're getting a chance to completely ignore any Reflex-based spell on a successful roll. So, to mitigate it, you're a little less likely to make that roll in the first place. From an IC perspective? It's still making you incredibly more reflexive than you would be otherwise. Compared to as you are without it, you wouldn't be able to completely jump out of the way of a Fireball. On your own and at your best, you'd still only get halfway out of it.
If I had to cook up a reasoning(Amia is full of back-reasonings at this point)... It's like hide armor, correct? Maybe it's giving you the reflexes of whatever "wary predator" it was skinned from. But it's like the armor has a mind of its own, and those magic reflexes are a bit slow to start up now that it's removed from the animal. There's a bit of
input lag. You go to jump when you see the Lightning Bolt coming your way, but there's a 0.1 sec delay before you do some sort of cat-like somersault to safety.
But yeah, it's there for balance from what I imagine. Maybe the thematics of it aren't the best, but it could be explained in a number of ways if you try. Whether the Reflex penalty is needed is more of a balance discussion than it is a matter of IC explanation, really.
Edit: In looking up what sort of animal it might have come from, the only mention of that name I found was this
this... Please tell me that Kumakawa armor is made by skinning ballet dancers.