Why getFrameSize() in ios 6.0 simulator 4 inch do not return 640*1136

I use this code to get the screen size in the retina 4-inch iOS 6.0 simulator

CCSize screenSize = CCEGLView::sharedOpenGLView()->getFrameSize();

It return 640*960,not 640*1136

But in iOS 4.3 simulator,it did return 640*960

Is it just a bug,would not effect the physic device performance?