SiblingIndex in uGUI

First questition: Is “1”(include child) always in front of"0"(include child) in my picture?

Second question: If first question is ture ,why when i change the label’s Y axis value (label named by “text???”),“1” 's child(the button’s text label) become front of “0”'s child? Is this A Bug ???

This really confused me…ps:unity version 5.3.1f1
[68832-qq截图20160425210959.png|68832]

i tried 5.3.4f1, this bug had been fixed~~~
" Fixed sorting issue where gird based depth sorting would fail to recognize overlapping unbatchable items. (770804)"may be this one?
http://unity3d.com/cn/unity/whats-new/unity-5.3.4