Stage.stageHeight in iPhone4 is halfed?

I am accessing a MC’s stageHeight property, the Stage’s height is supposed to be 640 (landscape, iPhone4), but instead it keeps returning 320.

The movie’s root stage has set to be 960x640 already, and all other sizes seems to be fine.
(I am testing a touch event’s stage position , e.stageY, and it’s correctly in the 960x640 scale)

It takes quite a long time to export an ipa so I haven’t tested this extensively; just wonder if anyone know what’s happening here??