DescriptorMatcher<"BruteForce">::knnMatch() returns too many matches (Bug #1795)

The test below asks for the best 3 matches to 2 train descriptors.
I wonder how the matcher finds 3 matches of 2 possible!
The test is added to 'nonfree' module accuracy suite.

TEST(Features2d_BruteForceDescriptorMatcher_knnMatch, regression)
{
    const int sz = 100;
    const int k = 3;

    Ptr<DescriptorExtractor> ext = DescriptorExtractor::create("SURF");
    ASSERT_TRUE(ext != NULL);

    Ptr<FeatureDetector> det = FeatureDetector::create("SURF");
    //"%YAML:1.0\nhessianThreshold: 8000.\noctaves: 3\noctaveLayers: 4\nupright: 0\n" 
    ASSERT_TRUE(det != NULL);

    Ptr<DescriptorMatcher> matcher = DescriptorMatcher::create("BruteForce");
    ASSERT_TRUE(matcher != NULL);

    Mat imgT(sz, sz, CV_8U, Scalar(255));
    line(imgT, Point(20, sz/2), Point(sz-21, sz/2), Scalar(100), 2);
    line(imgT, Point(sz/2, 20), Point(sz/2, sz-21), Scalar(100), 2);
    vector<KeyPoint> kpT;
    kpT.push_back( KeyPoint(50, 50, 16, 0, 20000, 1, -1) );
    kpT.push_back( KeyPoint(42, 42, 16, 160, 10000, 1, -1) );
    Mat descT;
    ext->compute(imgT, kpT, descT);

    Mat imgQ(sz, sz, CV_8U, Scalar(255));
    line(imgQ, Point(30, sz/2), Point(sz-31, sz/2), Scalar(100), 3);
    line(imgQ, Point(sz/2, 30), Point(sz/2, sz-31), Scalar(100), 3);
    vector<KeyPoint> kpQ;
    det->detect(imgQ, kpQ);
    Mat descQ;
    ext->compute(imgQ, kpQ, descQ);

    vector<vector<DMatch> > matches;

    matcher->knnMatch(descQ, descT, matches, k);

    //cout << "\nBest " << k << " matches to " << descT.rows << " train desc-s." << endl;
    ASSERT_EQ(descQ.rows, matches.size());
    for(int i = 0; i<matches.size(); i++)
    {
        //cout << "\nmatches[" << i << "].size()==" << matches[i].size() << endl;
        ASSERT_TRUE(min(k, descT.rows) >= matches[i].size());
        for(int j = 0; j<matches[i].size(); j++)
        {
            //cout << "\t" << matches[i][j].queryIdx << " -> " << matches[i][j].trainIdx << endl;
            ASSERT_EQ(matches[i][j].queryIdx, i);
        }
    }
}