Wrong measure on Zaxis on Translation Matrix of SolvePnP

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Hi, I am trying to get both rotation and translation matrix of a red square with webcam using SolvePnP. All the values are correct with my measures (with +-1mm of error) except the translation value of Z axis that gives me exacly half of the suppose value.

EDIT:

This is the code that give me the pose relatively to the center of the camera

void getPose(std::vector<std::vector<cv::Point> > contours)
{
    ///Variables
    cv::Mat rvec;
    cv::Mat tvec;
    std::vector<cv::Point2f> image_points;
    std::vector<cv::Point3f> object_points;
    std::vector<cv::Point> contours_center;
    std::vector<cv::Point2f> contoursf;

    for( int i = 0; i < 4; i++ )
        {
             contours_center.push_back(cv::Point(centercameraX-contours[0][i].x,centercameraY-contours[0][i].y));
        }

    std::cout << contours_center << std::endl;

    cv::Mat(contours_center).copyTo(contoursf);

    ///Model points in 3D
    object_points.push_back(cv::Point3f(25, 25,0));
    object_points.push_back(cv::Point3f(25, -25,0));
    object_points.push_back(cv::Point3f(-25, -25,0));
    object_points.push_back(cv::Point3f(-25, 25,0));

    ///Image points in the 2D image
    image_points.push_back(cv::Point2f(contoursf[0]));
    image_points.push_back(cv::Point2f(contoursf[1]));
    image_points.push_back(cv::Point2f(contoursf[2]));
    image_points.push_back(cv::Point2f(contoursf[3]));

    //Camera matrix obtained from camera calibration
    cv::Mat cam_matrix = cv::Mat(3,3,CV_32FC1,cv::Scalar::all(0));
    cam_matrix.at<float>(0,0) = 338.13;
    cam_matrix.at<float>(1,1) = 320.75;
    cam_matrix.at<float>(2,0) = 162.73;
    cam_matrix.at<float>(2,1) = 96.65;
    cam_matrix.at<float>(2,2) = 1;

    //Distortion matrix obtained from camera calibration
    cv::Mat dist_matrix = cv::Mat(1,5,CV_32FC1,cv::Scalar::all(0));
    dist_matrix.at<float>(0,0) = 0.0192;
    dist_matrix.at<float>(0,1) = -0.102;
    dist_matrix.at<float>(0,2) = -0.026;
    dist_matrix.at<float>(0,3) = -0.0026;
    dist_matrix.at<float>(0,4) = 0;

    ///Get rotation and translation matrix
    cv::solvePnP(cv::Mat(object_points),cv::Mat(image_points),cam_matrix,dist_matrix,rvec,tvec,false,cv::ITERATIVE);
    std::cout << "Rotaion: " << rvec << std::endl << std::endl;
    std::cout << "Translation: " << tvec << std::endl << std::endl << std::endl;
}

Result: Z distance in the translation axis is 158mm when it should be 320mm, the rest of values are correct

The green circle is the center of the image