Although I did not find a link to this in the docs, I am sure that the order of the elements in the array subviewscorresponds to their z-order. You can try to change the order of the subqueries in IB and log views on the console - you will see that the output supports this.
, : insertSubview:atIndex: exchangeSubviewAtIndex:withSubviewAtIndex:, z- subview, subviews...