IOS APP screenSize not correct

When i make an IOS App,and set

settings.enableRetina = true; // enables retina resolution if the device supports it.

in main.mm

and i use ofGetWidth() and ofGetHeight()

  void ofApp::draw(){
     ofBackground(0);
     randomPol.draw();

     ofDrawBitmapString(ofGetWidth(), 20, 20);
     ofDrawBitmapString(ofGetHeight(), 20, 80);
     ofDrawBitmapString("mouseX:" + ofToString(a) + " mouseY:" + ofToString(b), 20,140);


  // ofDrawBitmapString("mouseX:" + ofToString(mouseX) + " mouseY:" + ofToString(mouseY), 20,140);
ofDrawCircle(mouseX,mouseY,5);
}

I use iphone 6 plus, it's resolution is 1920 X 1080, why it will become 640 X 1136 ?Is there anyway i can change the setting?

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