DrawCircle in IOS will have offset BUG

I make simple example in IOS 9.0.

void ofApp::draw(){
    ofCircle(pos.x,pos.y,200,200);

}

//--------------------------------------------------------------
void ofApp::touchDown(ofTouchEventArgs & touch){

    pos.set(touch.x,touch.y);

}

//--------------------------------------------------------------
void ofApp::touchMoved(ofTouchEventArgs & touch){
    pos.set(touch.x,touch.y);

}

when i run this on my ipad and iphone.When i touch the screen,the circle is not in the center.It will offset,espacially near the edge of the screen.And when the circle size get bigger,it will offset more,It’s very strange?

When I use

float w = 400;
ofDrawRectangle(pos.x - w/2,pos.y - w/2,w,w);

the rectangle it’s ok,it will always in the center~

that could be an orientation issue, strangely enough.

I dumb dumb test this, so i just draw some shapes on the screen and check there isn’t anything weird going on. Also cout width and height