stage size with -platformsdk

I create a empty mobile project and just add an enter frame event to trace stage size,

then debug at iphone 6 plus,(physical device)

without -platformsdk,air gives stage size 1242x2208,that is correct.

when i give the -platformsdk value,air gives stage size 828x1472,

and both stage size returns different dpi.

I use air 20 and iOS sdk 9.3

is it  a bug of air? and any one who knows how to get the  1242x2208 with -platformsdk ?