video frame is different frome imgs?

edit

int main(int argc, char** argv)
{
    if (argc != 2) {
    help(argv);
    return 1;
    }
    std::string arg = argv[1];
    VideoCapture capture(arg); 
    if (!capture.isOpened()){ 
    capture.open(atoi(arg.c_str()));
    }
    if (!capture.isOpened()) {
    cerr << "Failed to open a video device or video file!\n" << endl;
    help(argv);
    return 1;
    }

    char filename[200];
    Mat frame1;
    Mat frame2;
    Mat frame3;
    Mat frame4;

    capture >> frame1;
    sprintf(filename,"frame%.3d.jpg",1);
    imwrite(filename,frame1);
    frame3 = imread(filename);

    capture.set(CV_CAP_PROP_POS_FRAMES, 100);

    capture >> frame2;
    sprintf(filename,"frame%.3d.jpg",2);
    imwrite(filename,frame2);
    frame4 = imread(filename);

    cout << (double)cmpHistOfHSV(frame1, frame2) << endl;
    cout << (double)partOfCmpHist(frame1, frame2) << endl;

    cout << cmpHistOfHSV(frame3, frame4) << endl;
    cout << partOfCmpHist(frame3, frame4) << endl;
    return 0;
}

the output:

0
0
4074.85
6790.25

why, the video frame is different the img?

Appendix：

double cmpHistOfHSV(Mat src_base1, Mat src_base2)
{
    const int HL_RANGE = 0;
    const int HR_RANGE = 256;
    const int SL_RANGE = 0;
    const int SR_RANGE = 180;

    const int H_BINS = 12;//50
    const int S_BINS = 6;//60

    Mat hsv_base1, hsv_base2;

    // Convert to HSV
    cvtColor(src_base1, hsv_base1, CV_BGR2HSV);
    cvtColor(src_base2, hsv_base2, CV_BGR2HSV);


    //30 bins for Hue, 32 for Saturation
    int h_bins = H_BINS;
    int s_bins = S_BINS;
    int histSize[] = {h_bins, s_bins};

    //hue varies from 0 to 256, saturation from 0 to 180
    float h_ranges[] = {HL_RANGE, HR_RANGE};
    float s_ranges[] = {SL_RANGE, SR_RANGE};
    const float *ranges[] = {h_ranges, s_ranges};

    //Use the 0th and 1th channels
    int channels[] = {0, 1};

    //Histogram
    MatND hist_base1;
    MatND hist_base2;

    //Calculate the histogram for HSV images
    calcHist(&hsv_base1, 1, channels, Mat(), hist_base1, 2, histSize, ranges, true, false);
    normalize(hist_base1, hist_base1, 0, 1, NORM_MINMAX, -1, Mat());

    calcHist(&hsv_base2, 1, channels, Mat(), hist_base2, 2, histSize, ranges, true, false);
    normalize(hist_base2, hist_base2, 0, 1, NORM_MINMAX, -1, Mat());

    double base_base = compareHist(hist_base1, hist_base2, 1);
    return base_base;
}