Bug of QSet

#include <QSet>
#include <QTime>
#include <QDebug>

int main() {
    int length1 = 1000000;
    int length2 = 1000;

    QSet<int> set1;
    for (int i = 0; i < length1; ++i) {
        set1.insert(qrand());
    }

    QSet<int> set2;
    for (int i = 0; i < length2; ++i) {
        set2.insert(qrand());
    }

    qDebug() << set1.size() << set2.size();

    QTime time;
    QSet<int> intersect = set1;
    time.start();
    intersect.intersect(set2);
    qDebug() << time.elapsed() << intersect.size();


    intersect = set2;
    time.restart();
    intersect.intersect(set1);
    qDebug() << time.elapsed() << intersect.size();
}