Item::childAt() returns an object at (x,y) while there is no any real object at (x,y)

import QtQuick 2.0
import QtQuick.Window 2.0

Window {
    visible: true
    color: "steelblue"

    Item {
        id: rect
        anchors.fill: parent

        Rectangle {
            color: "red"
            x: 0
            y: 0
            width: 16
            height: 10
        }
    }

    Component.onCompleted: {
        var found = 0;

        for(var i = 0;i < 18;i++)
        {
            var block = rect.childAt(i, 0);

            console.log("" + i + ": " + block);

            if(block !== null)
                found++;
        }

        console.log("Found " + found);
    }
}

qml: 0: QQuickRectangle(0x2abb2b8)
qml: 1: QQuickRectangle(0x2abb2b8)
qml: 2: QQuickRectangle(0x2abb2b8)
qml: 3: QQuickRectangle(0x2abb2b8)
qml: 4: QQuickRectangle(0x2abb2b8)
qml: 5: QQuickRectangle(0x2abb2b8)
qml: 6: QQuickRectangle(0x2abb2b8)
qml: 7: QQuickRectangle(0x2abb2b8)
qml: 8: QQuickRectangle(0x2abb2b8)
qml: 9: QQuickRectangle(0x2abb2b8)
qml: 10: QQuickRectangle(0x2abb2b8)
qml: 11: QQuickRectangle(0x2abb2b8)
qml: 12: QQuickRectangle(0x2abb2b8)
qml: 13: QQuickRectangle(0x2abb2b8)
qml: 14: QQuickRectangle(0x2abb2b8)
qml: 15: QQuickRectangle(0x2abb2b8)
qml: 16: QQuickRectangle(0x2abb2b8)
qml: 17: null
qml: Found 17