Real animation duration not equals to declared time

iimport QtQuick 2.4

Rectangle {
    id: root
    color: "red"
    anchors.fill: parent
    Rectangle {
        color: "green"; width: 50; height: 50;
        NumberAnimation on x {
            running: true; from: 0; to: 1000; duration: 5000;
            onStopped: {
                calculator.qmlAnimationElapsed();
                start();
            }
        }
    }

    Timer {
        interval: 1000; running: true; repeat: true
        onTriggered: calculator.qmlTimerElapsed();
    }
}

#include <QGuiApplication>
#include <QQuickView>
#include <QQmlContext>
#include <QElapsedTimer>
#include <QTimer>
#include <QDebug>

class Calculator : public QObject
{
    Q_OBJECT
public:
    explicit Calculator(QObject *parent = nullptr)
        : QObject(parent)
    {
        m_animation.start();
        m_timer.start();
    }

    Q_INVOKABLE void qmlAnimationElapsed()
    {
        qDebug() << "QML animation elapsed:" << m_animation.elapsed();
        m_animation.start();
    }

    Q_INVOKABLE void qmlTimerElapsed()
    {
        qDebug() << "QML timer elapsed:" << m_timer.elapsed();
        m_timer.start();
    }

private:
    QElapsedTimer m_animation;
    QElapsedTimer m_timer;
};

int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);

    QElapsedTimer elapsedTimer;
    QTimer timer;

    QGuiApplication::connect(&timer, &QTimer::timeout, [&elapsedTimer]() {
        qDebug() << "C++ timer elapsed:" << elapsedTimer.elapsed();
        elapsedTimer.start();
    });

    elapsedTimer.start();
    timer.start(1000);

    Calculator calculator;

    QQuickView view;
    view.rootContext()->setContextProperty("calculator", &calculator);
    view.setSource(QUrl(QStringLiteral("qrc:/main.qml")));
    view.setResizeMode(QQuickView::SizeRootObjectToView);
    view.showFullScreen();

    return app.exec();
}

#include "main.moc"