Wrong hit order of UIButtons

[fanling3]

Here is my setup:
UICamera z = -400
UIButton1 inside ScrollView, z = -120, with Box Collider, UIDrag Scroll View, UIPlaySound
UIButton2 z = -220, with Box Collider, UIPlaySound

Culling Mask of UICamera is Everything
Center of Z of all Box Colliders are 0
So according to the depth, UIButton2 should be triggered if UIButton1 and UIButton2 overlap, but the fact is UIButton1 triggered, why would this happen?

Thanks.

[ArenMook]

Z has no effect in NGUI 3. Everything is based on depth.