Why did kadabra evolve while holding everstone?

Question

It was definetly holding an everstone
And I tried this in both gen 4 and 5
Can you explain why

Answer 1

From Generation IV onward, the Everstone fails to prevent Kadabra from evolving.

Hope this helped!

Source

Comments

Why does it happen

---

I don't think there is a known reason, research I've found is just speculation

---

Why can't this work for Haunter :/