Clarification on conversion between RGB and CIELAB - opencv

Question

I'm working cum clouds of points with PCL. I recently had to convert the color information of the points that are in RGB to Cielab.
I have seen that it is possible to do with OpenCV and then I used the following code:
pcl::PointCloud<pcl::PointXYZLAB>::Ptr convert_rgb_to_lab_opencv(pcl::PointCloud<pcl::PointXYZRGB>::Ptr cloud) {
pcl::PointCloud <pcl::PointXYZLAB>::Ptr cloud_lab(new pcl::PointCloud <pcl::PointXYZLAB>);
cloud_lab->height = cloud->height;
cloud_lab->width = cloud->width;
for (pcl::PointCloud<pcl::PointXYZRGB>::iterator it = cloud->begin(); it != cloud->end(); it++) {
// Color conversion
cv::Mat pixel(1, 1, CV_8UC3, cv::Scalar(it->r, it->g, it->b));
cv::Mat temp;
cv::cvtColor(pixel, temp, CV_BGR2Lab);
pcl::PointXYZLAB point;
point.x = it->x;
point.y = it->y;
point.z = it->z;
point.L = temp.at<uchar>(0, 0);
point.a = temp.at<uchar>(0, 1);
point.b = temp.at<uchar>(0, 2);
cloud_lab->push_back(point);
}
return cloud_lab;
}
My question is: are the values I got correct? Should not LAB values be decimal and vary with negative numbers?
So I tried to do the conversion "manually" with the code available here.
When I visualized the two clouds in the CloudCompare I saw that they produced very similar views, even in the histogram.
Can someone explain to me why?