anchors.verticalCenter miscalculated with real height & y

import QtQuick 2.15
import QtQuick.Window 2.15
import QtQuick.Controls 2.15

Window {
    width: 640
    height: 480
    visible: true
    title: qsTr("Hello World")

    Rectangle {
        id:rect2
        width: 100
        height : 41.2
        color: "light blue"

        Rectangle {
            id:line2
            anchors.verticalCenter: parent.verticalCenter
            anchors.alignWhenCentered: false
            width: 100
            height: 0.9
            color: "black"
            onYChanged: console.log("Y, HEIGHT:", y, height)
        }
    }
}